This form of algae is also known as "green pea" soup. The water turns into a cloudy green colour.
The primary cause is excess ammonia coupled with strong light for extended periods. This may occur particularly in new tanks / planted tanks that are not biologically mature. It often results from over-feeding, high levels of livestock waste and/or plant decomposition. It is more common in very warm tanks (80f+).
For detailed steps on how to cycle a tank, click here.