I have had that nasty green water in my 55 gallon. (Most easy to notice when looking through the tank sideways and it also looks very green in a white bucket.) It was an algae bloom of large proportions! I tested my tank for phosphate and discovered there was an extreme amount of phosphate in the water. I believe it was due to a very high organic load in the tank. (many of my plants and surface living algae died during a parasite treatment.)
Anyway, the way I got the green water to go away was to first reduce the phosphate which I did through lots of water changes. I also bought a plec to clean up the algae growing on the plants, etc. Then I continued to do 15-20% water changes several times a week. AND, since my tank is in a sunny room, I close the shades while I'm away at work so the sun does not shine on the tank.
The water looks great now. But it did take some work and a couple of weeks. (I have nicely toned arms now from lugging 5 gallon buckets of water!)
Hope this helps--
sb