When I first purchased my lr, I noticed three stems sticking out of one of the rocks. As my tank matured one of them started sprouting leaves. I was pretty excited about it, and I guess my yellow tang was too. Two days after I introduced him into the tank, they disapeared. I guess I will never know for sure what was growing there. Good luck with your's though.
If they get little green balls on them it is probably grape caulerpa. There is a few post in the arch. about it. People say that it spreads like crazy and takes over your tank but my blue legs pick at it and it hasn't grown or shrunk in a few weeks. I like it.