I have what may be black algae all over the walls of my 30' round above-ground pool. I have used an algae killer for black algae and 7 bags of shock. I tried brushing the walls after my pool had cycled for three hours, but although the algae is coming off into the pool, the black spots remain on the walls. Will these dark spots go away, or will I have to get a new liner? Is this black algae or something else? Help, please!