It's cyano... fairly common in a young tank, especially if you happen to feed often. have you checked your phosphate levels?
Two things you can try: (1) put in a chaeto reactor (check out the pod hotel thread in DIY I posted -- it helps in my system and I get to have lots of pods for my mandarin ) , (2) you can use GFO. Less phophates and nutrients = less algae and cyano