I have a 35K play pool running SWG circupool. Filter is a triton sand filter with multiport valve. FAC 3.2 TAC 4.4 PH 7.5 ALK 115. Every now and then I will have a problem with what looks like to be foamy scum on the pool surface. It happens usually every other week and I think the filter sand is not the right type. I have attached a pic, not sure what the deal is.