Green water is caused by a planktonic (free floating) algae. Like all algae the cause of a bloom is excess nutrients (nitrate and phosphates.) With planktonic algae you have one good way to solve that problem. Simply by filtering you are harvesting and exporting the nutrients in the form of biomass. A good form of filtration is a Tridacna clam. A 100 micron sock filter changed out at least once a week is also very helpful.