looks like a hairy mushroom to me. I had one similar in my tank, I believed the store i bought it from called it a elephants ear (Rhodactis sp) mine was an uglier brown green color though.
Actually LFS. The reef tank they had at my LFS caught on fire (electrical) so they had to remove everything out and they were selling everything. Got the 8pound live rock with 20 of those hary mushrooms for 140$ Cdn.