That appears to be a green water algae bloom. To verify this put a little tank water into a white cup. If it looks green then it is a green water algae bloom. You have a couple choices in how to treat it. One is the black out method. This entails completely covering the tank, top, sides, bottom, so absolutely no light can enter the tank. Leave it like this for 3 days and NO peeking! This will break the black out cycle and you'll have to start all over again. This method can often work but if all the free floating algae isn't killed it will come back.
Your second choice is using a UV
sterilizer. This will usually kill the bloom off in about 24 hours and it won't come back. This is what many people on the forum have had good results using... Green Killing Machine Internal UV Sterilizer with Power Head at PETCO
Then once this problem is eradicated you need to only run lights for 6-8 hours. I'd start at 6 and each week add 1/2 hour to you photoperiod. Build up to 8 hours or so as long as you don't get algae growth in your tank.