This article is from the Aquaria: Disease, Algae and Snails FAQ, by Elaine Thompson, Thomas Sasala and George Booth
Green unicellular algae will sometimes reproduce so rapidly that the
water will turn green. This is commonly called an "algae bloom" and is
usually caused by too much light like direct sunlight.
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