if its a sponge, i had something similar, they grew, then just died out...every once in a wile one will pop up but it never caused any problems...
Thats what i have, like ten sprouted from that one and then they all dissapeared
While flipping through my Baensch Marine Atlas (anyone still have these?) the other day I came across a pic just like this in the Medusa worm section. It said the E. Nebulosa puts out its tentacles feeds then sheds them and fish love to eat them.
So if this occurs with any regularity or speed (sponge growth is slower than paint drying) then I'd vote on a worm over sponge.