I am noticing a recurring instance of what may be mustard algae even though my water is balanced, FC is appropriate for CYA, etc. I haven't noticed a particularly higher FC demand, but this yellow substance keeps coming back. Always in the same spot...deep end, the shaded wall. It does brush off, but requires more effort than I would think for algae. I have raised FC to mustard shock level and held it there until OCLT passed. Wonder what's going on here? Maybe it is not even algae?