(My first post here!) I have a 15,000 gallon inground gunite salt chlorinated pool, 10 years old, with white plaster. This summer for the 1st time there are tiny black spots mostly on the step surfaces. My water is crystal-clear. I also have greenish stains on the bottom of my pool so I hired a pool maintenance professional to test the greenish stains and said they are a mineral (iron I think.) He thinks the black dots are black algae, and he told me I should get rid of the black algae before he treats the stains. He recommended I buy the best algae killer (not maintenance, but killer) and brush them often and keep my chlorine level up over 5 until the spots are gone. I added the black algae killer - half a bottle, then 2 days later the other half of the bottle, and have kept the clorine level high. That was 2 weeks ago and I don't think any spots have disappeared. When I brush with the wire brush, I brush hard back and forth - no matter how much I brush or how hard I see no difference - nothing comes off as I brush and the next day it looks the same. I touch the black dots I can't feel anything, just plaster - it doesn't feel soft or smooth, and nothing comes off with my fingernail. It looks like someone used a black fine sharpie pen and marked tiny dots in my plaster! To describe, it looks like vanilla ice cream - you know how there are dark spots in vanilla, that's what my plaster on the steps looks like. I'm wondering if I should buy some chlorine pucks or trichlor pucks and see if that does anything to the dots? Maybe it isn't black algae? What do you recommend?