If the Rock is that new, and you don't like the looks of it, take the rock back out and scrub it off in some tank water. No harm no foul. The Sponge wont' hurt anything either.
"May" be a folded up mushroom. Does it change shape? It is hard to tell.. If it is a sroom (mushroom) you got a freebie.
Also a sroom would be slimy and could be purpleish. If it is in fairly good light it should flatten out at some point, if it is a sroom.
EDIT: wait.. I just caught the 3 days bit.. I doubt it is a sroom.
Sponges are filter feeders.... you like filter feeders. You want to collect all the filter feeders you can muster. If you have a refugium it can live there.. I'd leave it in the tank if you don't have a refugium.