Mold vs. Mildew on a Gulf Coast House: You Are Usually Looking at Neither
The mold-or-mildew question is borrowed from indoor restoration writing, and outdoors it answers almost nothing. Four different things discolour a Florida exterior. Only one of them is a fungus, one of them is not alive at all, and the difference decides whether you are buying a wash or chasing a leak.
Most of what a Gulf Coast homeowner calls mold or mildew outdoors is algae. The black streaking on a roof is Gloeocapsa magma, which the shingle manufacturers’ own association says “may be mistaken for fungus growth, soot, dirt, moss, or tree droppings” and reports is “most widespread in the Gulf States.” The chalky white bloom on stucco is not an organism at all. True mold — a fungus — is usually the dark speckling on shaded painted wood, soffits and siding, and the EPA’s rule for it is about water rather than cleaner: “The key to mold control is moisture control.” Colour will not sort them out for you, because “molds come in many colors; both white and black molds are shown here.” Use the field key below, then answer one question: is the water on the surface, or in the wall?
Every crew on this coast is asked the same question in the same words, usually with a phone held up to a north-facing wall: is that mold or mildew? It is a reasonable question and it comes from a real place — there is an enormous amount of writing about the difference. Almost all of it is about the inside of a house.
That genre is written by indoor restoration companies, and it is written about drywall, bathrooms, basements and crawlspaces. Transplanted onto the outside of a Florida house it quietly does two unhelpful things. It implies that whatever is on your stucco is an emergency, and it implies that mold and mildew are the two candidates. Outdoors, on this coast, they usually are not.
What the EPA actually says about mold that is outdoors
Start with the sentence the indoor genre leaves out. In its brief guide to mold, moisture and your home, the EPA writes that “molds are part of the natural environment. Outdoors, molds play a part in nature by breaking down dead organic matter such as fallen leaves and dead trees, but indoors, mold growth should be avoided.”
Read the sentence as written. The concern, the ten-square-foot threshold, the protective equipment, the containment — all of that is framed around indoor growth, because indoors is where the spores and the people share air. Mold on the shaded face of a soffit is not the same event as mold behind a bathroom vanity, and treating it as one is how a homeowner ends up frightened about a cosmetic problem or, worse, reassured about a structural one.
The other half of that same guidance is the part worth taking seriously, and it is not about cleaner: “The key to mold control is moisture control… There are many types of mold, and none of them will grow without water or moisture.” That single line is what makes the rest of this page useful, because it turns identification into a decision rather than trivia.
And “mildew” has a real meaning that is not a wall
Here is the part that surprises people who have used the word their whole lives. Mildew is a genuine, specific thing in plant pathology, and in a Florida landscape you can go outside and find it. UF/IFAS describes powdery mildew as “a serious fungal disease that attacks a wide range of plants, from squash to roses,” whose “symptoms are unmistakable: spots or patches of white powder, usually on the leaves.” Crapemyrtles, which are planted along half the streets on this coast, are “especially susceptible” — the culprit there being the fungus Erysiphe lagerstroemiae.
So the mildew on your crapemyrtle is mildew. The “mildew” on your soffit is a household nickname for mold, and the “mildew” on your roof is not a fungus at all. None of that is pedantry once you notice that the three of them respond to completely different treatment, on completely different schedules, at completely different prices.
The field key
Work down this table from what you can actually see. It covers effectively everything that discolours an exterior on the Gulf Coast.
| What you are looking at | What it actually is | What that changes |
| Brown-to-black streaks running down a roof slope, worst on the shaded side | Algae — a cyanobacterium, most often Gloeocapsa magma. Not a fungus. | Soft-wash chemistry with a timed dwell. Never a machine. It will recur. |
| Flat green or grey-green film on north walls, pavers, concrete, screen frames | Green algae, growing in the pores of the surface | Dilute cleaner plus dwell time; the fix is drying the surface out, not scrubbing harder. |
| Dark speckling and blotching on shaded painted wood, soffits, fascia and vinyl | Mold — an actual fungus, and the only thing on this list that is | Wash it, then find the water. If you skip the second half it returns. |
| Chalky white powder or bloom on stucco, block, pavers or a retaining wall | Efflorescence — mineral salt carried out of the masonry by water. Not alive. | Bleach does nothing to it. Different job entirely. |
| Raised, crusty, grey-green rosettes gripping a roof or a wall | Lichen — a fungus and an alga living as one organism | The hardest of the group to remove and the most likely to leave a mark. |
Why so much of it is algae here specifically
This is not a regional boast, it is in the trade literature. The Asphalt Roofing Manufacturers Association’s bulletin on algae discoloration of roofs states that the staining “usually has a brown to black appearance and may be mistaken for fungus growth, soot, dirt, moss, or tree droppings,” that “Gloeocapsa magma is probably the most prevalent of several algae species that contribute to discoloration,” and — the line that matters for anyone reading this from Lee, Collier, Sarasota or Escambia County — that it is “most widespread in the Gulf States and along the Northwest and Eastern Seaboards… especially those subjected to warm, humid conditions.”
ARMA also notes that “almost all types of roofing systems are susceptible,” which is why tile roofs in Naples and shingle roofs in Pensacola both streak, and that the discoloration “should not be confused with moss or tree droppings, which typically produce only localized discoloration” — patchy and local rather than running down the slope. We cover the organism itself, and why rain makes it streak vertically, on the roof-streak page; the point here is only that the most common dark stain on a Gulf Coast house is the one thing on the list nobody calls by its right name.
Two practical consequences come straight from the same bulletin, and they are worth knowing before you accept a quote. The dwell is specified: the solution “should be left on the roof for at least 15 minutes but no more than 20 minutes” before a gentle rinse. And the honest expectation is set by ARMA rather than by us: “the effectiveness of such cleaning is only temporary, and the discoloration may recur.” A crew promising a permanent result on a Gulf Coast roof is promising something the manufacturers’ own association does not.
The question that actually decides the job: is the water on it, or in it?
Once you know which of the five you have, there is exactly one more question, and it is the one that separates a two-hour cleaning visit from a repair. Run this checklist against the stain.
- It is only on the outside face, and it follows shade. Worst on the north elevation, under a canopy, behind a hedge, along the drip line. This is a cleaning job. The growth is living on the surface and on the moisture the climate supplies.
- It follows a sprinkler arc or an irrigation overspray pattern. Still a cleaning job, but it will come back on a short cycle until a head is re-aimed. Free fix, and it beats any cleaner.
- It runs in a narrow vertical band under a gutter joint, a window head, a hose bib or an AC condensate line. That is not a climate stain, that is a leak drawing its own picture on your wall. Wash it if you like; it returns on the same line every time until the fitting is fixed.
- It appears on the inside face of the same wall, or you can smell it indoors. Stop. This is no longer an exterior-cleaning question and no pressure-washing crew should tell you otherwise. The EPA’s mold cleanup guidance is the right starting point, and it puts the do-it-yourself boundary at about ten square feet of affected area before you should be calling a remediation contractor.
- It is white, powdery, and it came out of the masonry rather than landing on it. Efflorescence — a mineral problem with a moisture cause, covered in the stucco guide. Sodium hypochlorite is completely inert against it.
The EPA states the consequence of getting this wrong in one sentence: “If you clean up the mold, but don’t fix the water problem, then, most likely, the mold problem will come back.” Outdoors that translates cleanly. If the stain has a source, the wash is a countdown, not a cure — and you will conclude the crew did a bad job when in fact they cleaned a wall that is still being watered.
Colour will not identify anything, and neither will the word on the bottle
Two shortcuts to abandon. The first is colour. Black is widely assumed to mean the serious kind and green or white the harmless kind, and the EPA disposes of that in a photo caption: “molds come in many colors; both white and black molds are shown here.” On an exterior the colour tells you more about the substrate and the light than about the organism.
The second is the label. A bottle marketed as a mildew remover and a bottle marketed as an algae remover are frequently the same dilute sodium hypochlorite with a surfactant, and that chemistry does work on all three of the living stains here — but it works because of contact time, not concentration, which is the whole argument of soft washing versus pressure washing. It also does nothing whatsoever to efflorescence or to rust, which is why the rust page exists separately. Matching the bottle to the organism matters far more than the strength of the bottle.
The one place this becomes real money: paint
If you are within a year of repainting, identification stops being academic. The EPA’s cleanup guidance is blunt about sequence: “Do not paint or caulk moldy surfaces. Clean up the mold and dry the surfaces before painting. Paint applied over moldy surfaces is likely to peel.”
That is the reason a painter insists on a wash first and it is not an upsell — it is the difference between a paint job that lasts a Gulf Coast decade and one that lifts in sheets off the shaded elevation in three summers. Note also the honest limit in the same document: “mold may cause staining and cosmetic damage. It may not be possible to clean an item so that its original appearance is restored.” On painted wood that has carried growth for years, a wash removes the organism and may still leave a ghost. That is a paint problem at that point, not a washing failure, and a crew that tells you so before they start is being straight with you.
If you would rather have someone else stand in front of the wall and name it, the Pensacola crew we send painted-wood and soffit work to is where we point Panhandle homeowners — that metro runs to wood siding and shingle rather than the stucco and tile of the southwest coast, so it is the one where the true-fungus half of this page comes up most. How often any of it needs repeating is a separate and more useful question, and we answer it by surface in the coastal cleaning-interval guide.