I had this very same problem last year, it turned out to be dead algae, so fine the filter could not filter it, I added all kinds of stuff ,nothing worked. the only solution is clean water.
I had to empty the pool,and scrub it down with Clorox. Worth every bit the energy it took, because my water has been perfect this year (so far) another thing, how old is the sand in your filter?
good luck to you