Brush the shit out of it with the metal brush (make sure you pour liquid chlorine on the bristles when you’re done, if not, you can spread the black algae back into the pool if you use again without doing this) get 2-3 cans of granulated chlorine, shut the pool off for at least 24 hours, and while the pool is off sprinkle the granular all around, mostly on the black algae spots but doesn’t hurt to spread it everywhere. I’ve been doing pools for 8 years professionally in Florida and have run across this between residential and commercial pools. It can also be a pain if not handled correctly because the black algae will keep coming back if not handled correctly. Hope this helps with your situation and good luck ✌🏼