Hi, I uncovered our pool and found we have this yellow/green sand-like stuff on the sides and bottom of Viking Fiberglass pool. I treated for algae and a lot settled to the bottom like 'dust'. This is not like anything I have ever seen. I can't 'brush it off. It is like scale. The stuff is on the sides---near the water line I tried to use a scotch bright pad to clean it off. It worked somewhat, but the stuff likes to be SCRAPED off! I used my finger nail, and it took it off back to the gel coat-nice. BUT I cant scrape the whole pool wityh my fingernail. It feel like our fiberglass pool is morphing into gunite! and ROUGH. SWhat is this and how do we get rid of it? I thought maybe Ascorbic Acid? Please...help me! Thanks