That is a pretty serious green water algae bloom. Excess lighting left on too long with high nutrients level are just a couple reasons algae blooms can happen. Anyway you have 2 choices. You can first try a 3 day black out. Literally cover the top, bottom, and sides of the tank so no light can penetrate in. No peeking or you break the black out cycle. Open the tank up on day 4 and do at least a 50-60% WC. Only run lighting 6 hours daily for a while and be sure to do weekly 50% WC's to keep nutrient levels in the tank low. If it's near a window getting a lot of sunlight find a way to block it from the tank. This method sometimes work but if all the free floating algae isn't killed it's likely the bloom will return.
The second method which will work is using a UV sterilizer. Many people on this forum have gotten very good results using this...
Green Killing Machine Internal UV Sterilizer with Power Head at PETCO. This will kill the free floating algae within a couple days. Afterwards do a large WC to remove dead algae so an ammonia spike doesn't occur in the tank.