I am almost certain that it is a brown/lavender star polyp. I have some in my tank, it's very much like Green Star Polyp, but when its polyps are contracted, the surface of the coenenchyme is smooth instead of knobbly. Yours looks exactly like whats in my tank, and the only time i have really seen it for sale was when i bought it, but i agree with KrazeKajin, it should have grown much more by now. I believe its scientific name is: Erythropodium caribaeoreum.
Good Luck!