You may as well forget trying to explain the sponge to Turbo. Sounds like he/she is determined to remove this thing from the tank.
Turbo, even dry rock will sometimes have a hitch hiker make it into your tank. My dry rock that I through in just for some aqua scaping ended up have a couple of small feather duster pop open once the water was right. I figure it was just a hidden benefit since they are filter feeders. Rather than trying to pick the sponge off, maybe it's not too late and it hasn't sent out little seeds for reproduction, tank the rock out and wash/soak it in fresh water. That might/should kill any saltwater life remaining on the rock. Just my 2 cents.
Turbo, even dry rock will sometimes have a hitch hiker make it into your tank. My dry rock that I through in just for some aqua scaping ended up have a couple of small feather duster pop open once the water was right. I figure it was just a hidden benefit since they are filter feeders. Rather than trying to pick the sponge off, maybe it's not too late and it hasn't sent out little seeds for reproduction, tank the rock out and wash/soak it in fresh water. That might/should kill any saltwater life remaining on the rock. Just my 2 cents.