Brown spots

[Yelimex]

There a few brown/yellowish spots on my pool. When I brush them, it lifts and looks like sand but it only end up landing on a different spot and can’t get rid of it. Any sugg?

FC 6
TA 90
pH 7.8
CYA 50

 

[Texas Splash]

Maybe it's just me, but I can't see anything well in the pics. Have you added anything unusual to the water lately? Have you performed an Overnight Chlorine Loss Test? When was the last time you pulled the carts to clean the filters? How do they look?