It could be a lot of things that can be the issue but for right now I think dosing your tank is the best bet. It could be that the lights are on for too long, over feeding, high phosphate levels.... I suggest dose ur tank, do a water change of 25%, cut back a couple of hours on your lighting, and if you feed ur tank 3 times a day bring it to once a day. At one point I would feed only every other day. Also do yourself a favor and get some cheato (aka spaghetti algae)