Sounds exactly like what I was experiencing earlier this summer with mustard algae. I didn't get rid of it until I shocked to 25-30 PPM (cya = 80) and held it there for over 24 hrs with the pump on around the clock. I definitely vacuumed to waste since the sand filter wouldn't catch it. I followed that with PolyQuat 60 and I have had pristine water ever since.

Good luck!