If you brush vigorously and it doesn't go away it's something else. I've used the trichlor tablet scrub very effectively for stubborn algae. Careful with the acid approach, you can eat up a lot of plaster very quickly. In case you can't get it off with black algae treatments we'll get a stain expert to weigh in. @Swampwoman .Really hard to say... honestly brushing doesn’t touch it so for weeks I’ve just assumed it was stained plaster... but then someone on a local Facebook group had something similar but worse and everyone was saying black algae... it doesn’t “look”like algae to me as far as what I’m used to mold looking like..
To test I took a pvc pipe and put it over a spot and poured acid in and let it sit for like 20 seconds and it went away... didn’t work on a second spot... obviously this approach won’t work on some of the angles and stuff plus I don’t wanna put too much acid in.
So thanks for the help guys... would you think just getting a couple gallons of chlorine and dumping them in is the right move here, opposed to just setting the generator high and waiting? Not sure how fast it reacts to setting changes.