how strange, ever had salt water in there, or perhaps rock that was originally from a marine tank?
other than that it could just be a bacterial organism that resembles a star fish.
i had a marine reef tank a few years ago and you would be suprised the amount of random creatures and corals that would appear from nowhere because of the plankton layer in marine tanks.
Do have anymore pics? It is hard to tell what if is.
no sorry, i cant seem to find whatever it was now.