Hay Bwood
It is an algae bloom, very common stuff. Strangely enough it is called green water. I assume this is an unplanted tank.
A Green water algae bloom is caused by excess nutrients in the water column which are coming from...
1. Too many fish in the tank therefore excess urea (fish poo).
2. Over feeding and again excess urea from decomposing food.
3. 1 and 2 plus too much light.
That is pretty much it. It is a pain to get rid of.
A diatom filter will cure it in about 5 days
A UV Sterilizer will kill it in two days
It will come right back if you don't fix the problem. (see 123 above)
Here is what I would do.
Find the food source for the algae and stop doing what ever that is.
Do a 50% water change with a good vacuuming of the gravel.
Do a 3 Day complete blackout (plastic garbage bags, no light, no peeking) your fish will be fine.
When you unwrap the tank do another 50% water change.
Regards,
Jay
