Hello! I've fiberglass pool for two years and everyrhing was excellent. But three month ago all the surface under water started covering with white stains (not even stains but fur). I can't get rid of it with ordinary chemistry. At first I thought it was a calcium, tried Decalcit Super, no results. water testing results show normal level of Ca, Na, Mg. When surface is wet pool's colour nearly normal but when it's dry it's white.
What can it be & how to get rid of it?