I have an in-ground pool with a vinyl liner and I noticed small black dots forming on the liner. At first, they only appeared in the shallow end (which gets more shade) but they have now spread to the deep end. They are stubborn and don’t come off with shocking or regular brushing. They appear to be hard and stuck-on, unlike a sticky film. I have tried stain remover, rubbing a chlorine puck, etc. I figure it is either black algae (although I read that black algae is uncommon with vinyl liners) or mineral deposits, but nothing I try to remove them seems to make a difference. Any ideas?