What are your readings? Ideally use a good test kit like the Taylor 2006 — need to know them all. Especially cya and chlorine. That looks like algae possibly? I’m no expert


Hard to tell from the picture but it looks like black algae. If so then it will take a bit of work to get rid of. Might want to read up on it to confirm if it is black algae. https://www.swimuniversity.com/black-algae-pool/


Ah my friend welcome to the black algae club. If you want to take care of this the easy way, you either need to have scuba gear or hold your breath easily. Get a bunch of tricolor pucks and then go under with em. Scrub the plaster with and the algae should come off. The tricolor puck scrubs the slime layer off and also hyperchlorinates the immediate local area around the black algae. I was brushing and shocking my pool for days till I said fuck this and get the tricolor out and scuba dove my pool for 4 hours. Looks great now.