If you’re wondering whether your concrete has dirt or algae, look at how it behaves. Dirt usually smears brown or gray when you wipe it, algae leaves a greenish film or powder in damp shade, and mildew tends to look flat and dusty.
Color by itself won’t tell you what’s on the slab. Skip the harsh-chemical, high-pressure approach until you know what you’re removing. If you check what transfers to a paper towel and where the slab stays wet in Wilmington-area humidity, you can read the slab like tire tracks in driveway dust and pick a cleaning approach that lasts.
60-Second Concrete Spot Check
You scrub, rinse, and it looks better for a day, then the same dark patch reappears like you never touched it. Before you buy another cleaner or crank up the pressure washer, get a quick read on what you’re dealing with.
Begin with a one-minute test that confirms what’s on the surface. Wet a 6-inch patch, wait 30 seconds, and then wipe hard with a white paper towel. If it lifts as brown/gray smear and the spot looks normal when damp, you’re mostly dealing with dirt. If it lifts as a greenish film or powder and the area lives in shade, algae is the likely driver. This is true even if the concrete “doesn’t have soil on it,” or it reminds you of Wet & Forget Outdoor results.
Next, use feel and transfer, since mildew shows up in texture more than hue. If black/gray looks flat and dusty and wipes off, that points more to mildew than tracked-in grime. If nothing transfers and it comes back fastest where water lingers, treat moisture location as part of the ID and adjust your cleaning plan accordingly.
In coastal humidity, algae and mildew can keep reappearing anywhere moisture lingers—even after you’ve cleaned the surface. Read more in our article: Roof Algae Causes Coastal Nc
Dirt vs Algae vs Mildew on Concrete: The Tells
A homeowner in a shady corner of the driveway keeps treating a greenish cast like tracked-in grime, and it keeps coming back after every rain. The giveaway isn’t the shade of the stain; it’s what lifts and where the slab never quite dries.
If you’re staring at “dark concrete,” don’t rinse and repeat guesses by color alone—concrete discoloration causes aren’t always obvious. What matters is whether it behaves like a deposit sitting on top or like staining embedded in pores. Those pores act like a sponge, and they hold onto dirt. Along the North Carolina coast, shade plus humidity keeps surfaces wet longer, which is why algae tends to reappear in the same spots.
| What it is | Paper-towel transfer | Look/feel | Where it shows up most |
|---|---|---|---|
| Dirt | Brown/gray smear | Often looks “cleaner” when wet | Traffic paths |
| Algae | Greenish film or powder | Thin film/powder | Damp shade (edges, low spots, sprinkler overspray) |
| Mildew | Fine dusty residue | Flat, dusty dark spotting; less slick than algae | Recurring patches in humid, slow-drying areas |
Older, rougher concrete often “goes green” faster even when you’re on top of maintenance, which aligns with findings that rougher surfaces can encourage faster algal colonisation than smooth ones (see: ScienceDirect).
Black or green growth on exterior surfaces is usually a moisture-management problem first and a cleaning problem second. Read more in our article: Mold Algae Black Streaks
What to Do After You Identify It
When you match the cleaner to the culprit and fix the damp spot that feeds it, the concrete stays cleaner longer with less effort. The goal is one solid clean-and-dry cycle, not a weekend ritual.
If it’s dirt, start with a stiff brush, a concrete-safe detergent, and a thorough rinse. If it’s algae, use an exterior cleaner labeled for algae and give it dwell time, then scrub and rinse; don’t treat it like “just grime,” because it’ll return if the slab stays shaded and wet. If it looks like mildew, clean it like a surface deposit, then focus on drying conditions.
Pressure washing is wildly overrated for most slabs—especially in the pressure washing vs soft washing concrete debate. On older or rough concrete, aggressive pressure can etch lines and open pores that grab more gunk later, no matter what the Home Depot or Lowe’s pressure washer aisle PSI/GPM talk says. Fix what keeps it damp (sprinkler overspray or downspout discharge). Call a Wilmington-area pro for concrete cleaning Wilmington NC if the staining doesn’t lift after a proper clean-and-dry cycle or the surface is spalling or soft.
A softer, dwell-time-based wash is often safer than blasting when you’re trying to remove organic growth without roughening the surface. Read more in our article: Soft Wash Vs Pressure Washing
Roof not getting any younger? Contact us at Contact us or call 910-241-1152 to find out where you stand.
