Sounds like you're dealing with some sort of cyanobacteria. As reef dude mentioned, there are a couple of chemical responses you can use to combat it. But what's really important it track down the cause.
On another forum I chat on, a bunch of us came to realize we had one thing in common. We all used San Fransico Bay
brine shrimp. I switched to a different brand as a result.
There are 2 things out there that I'm aware of that are effective in combating cyano and I've used both:
http://www.marinedepot.com/Boyd_Chem...FIMERM-vi.html
Very effective. Will keep cyano away for a good 2 to 3 months. Usage: turn off protein skimmer, UV and remove carbon. Add airstone to be safe. Treat. Do a 20% water change 2 days later. Add fresh carbon and watch the skimmer...it will foam and cause micro bubbles.
http://www.drsfostersmith.com/produc...fm?pcatid=4121
Not as effective. I'm on my 3 treatment. However you don't have to remove anything though I did remove my carbon. Skimmer will go crazy and foam and produce micro bubbles. It'll need to be adjusted. No water change to do.
If you have the ability to deal with the water change and can keep O2 levels up for a few days, I'd use the Chemi-Clean. After 2 days, it'll be pretty much gone. A week later everything is back to normal.