Green Floating Algae
-caused by excessive lighting and high nitrates and phosphates
You'll need a blackout from 3 days to a week by covering your whole tank and not allow any penetration of light.
UV sterilizers will also work. And so will a bunch of daphnia(if your tank has no fish).
Pls take note, green floating algae can kill your fish by consuming at night the oxygen thus suffocating the fish. Not only that, if the algae die, much CO2 will be released killing the fish.