I think that it probably came in with the live rock. If so, I've had it since June or August of 2004. I was looking thru Calfo and Fenner's "Reef Invertebrates" last night and I am wondering if it maybe is some sort of Holothurid (Sea Cucumber) I remember a slug sort of thing that was attached to the live rock when I put the rock in the tank. At the time it was wedged into the rock and sort of felt like dried silicone. After the rock was submerged, it expanded and moved off the rockwork and disappeared. The live rock, by the way, came from Liverocks.com. It could be some sort of anemone, but my gut feeling is that it is something else.