thats interesting, i think thats coral but thats from me. take that picture back to the store and ask them about it. keep me updated i wanaa know! haha good luck
Looks like it could be a calcerous form of macro algae that is trying to make a comeback? Hard to say, there is so much random stuff that comes on live rock.
Neomeris annulata. It's a macro algae that some people love and want to keep it and others hate it. I've seen this really take off in some tanks and just wither and die in others, relatively difficult to keep but can take over a tank if it likes the params. Here's a picture of it thriving....Melevsreef.com - Neomeris annulata