Increased light levels will usually make it disappear. Easily removed by wiping the glass or siphon vacuuming the affected area. Green unicellular algae will sometimes reproduce so rapidly that the water will turn green. An algae bloom can be removed by filtering with micron cartridges or diatom filters. UV sterilizers can prevent the bloom in the first place.
Green water is very useful in the raising of daphnia and brine shrimp.
Film algae Grows on the aquarium glass and forms a thin haze. Easily removed by wiping the glass. Considered normal with the higher light levels needed for good plant growth. Spot algae Grows in thin, hard, circular, bright green spots, usually on the aquarium glass but also on plants under high light conditions. Considered normal for planted tanks.