Is your fill water especially hard ???

If the water is milky cloudy, then your high CL levels could be causing some percipitate to come out of the water, and the fact that you have to keep adding so much, keeps refreshing the supply. Endless circle.
With CC=0 then is seems unlikley that you have an algae problem.

Are you using a granular shock ? ... these have a bit of a reputation to cause milkiness.