Hard to tell by the pic, but my guess is that it is the beginning of hair algae. This is really common in new setups. Astria and turbo grazers will keep it down, as will zebrasoma tangs (yellow, sailfin, etc), hermit crabs, and urchins. If it gets long, it's easy to remove strands by hand, but a PITA.
My recommendations would be:
1. Add grazers, especially snails.
2. Dose with calcium supplements to get the coraline algae going. This will out-compete the green stuff.
3. I don't know how much light you are giving it, but coralines don't need much. Not turning the light on may be enough, but definitely don't give it full reef light, otherwise you will end up with green soup.
This is a great article dealing mainly with making your own live rock, but also goes on to curing and growing coralines. The
site is chock full of good reading, and I highly recommend checking it out.