Cyanobacteria?

[T-Dizzle777]

I have just set up a salt water Bio Cube 29, previously owned. I saw the tank set up, and he disassembled the tank the day after, putting the live rock in some water so as not to kill it. The live rock seems to be doing great, and I've got some hair algae growing, as well as some other living organisms that I can see (clams etc.). The problem I am having is I am getting decent portions of cyanobacteria growing in the tank. I have a molly in there to test the water and the molly is doing fine. I've just recently set up the skimmer and I've got a power head growing. I am also going to do a water change tonight. I have also been limiting the amount of light to about 4 hours or less day, today for only 1 or 2. Can anyone think of what I am doing wrong or what I can do manage it?

The tank's been set up for about 3 weeks.