I have a bunch of black dots on my vinyl liner. They first appeared just in the shallow end (which is a lot shadier) but have spread to the deep end. They’re quite stuck-on and hard to the touch – not sticky, fuzzy, or slimy. They don’t come off with regular brushing. I think I generally practice good pool maintenance: water gets tested, use enough chlorine, etc. My research has narrowed it down to two most likely causes: black algae or mineral deposits. However, I read that black algae is rare in vinyl liner pools as it is not as porous. Any ideas what the black dots could be and how to get rid of them and prevent them from coming back?