That dark streak below your AC condensate line isn't dirt — it's algae growing inside the pipe

A central air conditioner doesn't just cool air — it condenses the moisture out of it, and that water has to go somewhere. It runs out through a condensate drain line, usually a small PVC pipe that exits near the outdoor unit or along an exterior wall. Florida's AC systems run long hours for most of the year, so that line stays warm and wet almost constantly — conditions organic growth thrives in.

Algae and mold can colonize the inside of that line the same way they colonize a damp shower drain, and once a slimy buildup forms, bits of it wash out with the condensate and streak down the wall or stucco below the pipe's opening. It's easy to mistake for ordinary grime, or even for the dark roof algae that streaks Florida shingles, but it's a different organism coming from a different source — and it keeps reappearing because the line is still shedding more of it every time the AC runs.

That's also why wiping the wall clean doesn't make it stay clean. The stain is a symptom of what's built up inside the drain line, not just what landed on the surface, so it returns on its own schedule as long as the source keeps feeding it.