I *think* I have this same stuff in my tank. It will grow in small clumps and then after awhile it will fall off the rock ( gets blown off by a powerhead I'm guessing ). I don't think its grown more than 2 inches, maybe less. Everytime it comes off the rock, a new piece will grow in the same place. I've started to see small chunks in the surrounding area too. It doesn't see to have caused any harm to far, but aiptasia looks innocent too in the beginnings. I've had it at least 6 months in my tank.
I've always wondered what type of algae it was also. I am assuming its algae, but not certain.