I keep getting some little 1/4" round black algae spots in my pool. My numbers seem good and I'm not consuming chlorine overnight, but these little buggers keep showing up. Fairly sure they're some type of algae, they will scrub off, but takes a little effort with a stiff brush. If you look closely at them they do have a green tinge, but from a few feet away they look black. They tend to stick in the usual algae hangouts. Ridges in the plaster, corners, etc... This isn't a major outbreak, but I can't figure out why they keep coming back.
Any thoughts on what type of algae this is? Not mustard right? Why is 6.5 FC not killing this stuff and letting it regrow?
CYA: 35 (will bring up to ~50-60)