Vinyl. It's only in this one corner of the pool too. Not sure if some foliage maybe got under the pool cover and sat here for 6 months or what it could possibly be. Figured scrubbing it directly with chlorine might do something but no dice.is that pool plaster or is it vinyl.
Yeah, smearing brown -- was hard to get any up, but scraped some off the step with a putty knife and the poof was brown and where I rubbed the white paper on the step was brown.Do like the article says -- scrape up some and smear it on a white paper. If it smears greenish black, it's black algae.