I did a search for this problem on TPF with no results so:
My sister in law and I have fiberglass pools with a chalky film on the sides, below the water level. It wipes off easily and clouds the water with swimmers stiring it up. Hers is worse than mine and the pools are about 8 yrs old.
Readings are similar.
Is the glass getting old and chalking like boats do?
Any ideas on cause and fix?
We also have a few small black stains that look like they fan out from the center in one direction. About pencil eraser size.