I keep getting white circular clumps on the bottom of the pool. When you brush them they dissolve into a powder. Chlorine numbers are good and show no combined chlorine. I brushed/vacuumed and brought the pool up to shock level anyway to see if it would help. Passed the overnight loss test and still showed no combined chlorine, but the spots came back. So its not algae, right? My calcium hardness test is not high (about 60), so could it be calcium deposits? We do have metal in our water and use sequestrant for that. The pH was high at about 8, but I have brought that down to 7.4 with MA. I vacuumed to waste today (3 times) and brushed and the bottom is clear at the moment, but I don't know what else to do if it comes back again. WHAT IS THIS STUFF??