The thing about sponges is that it can reproduce itself better than anything else you could ever find, if you blend a sponge every cell can grow into a new sponge. If you scrape it off in your tank each of the flecks of sponge that float away are potentially another sponge now somewhere else in the tank. If I were you I would take it out, get some new/old SW and put it in there. After that scrape and cut as much as you physically can from the rock. Then you will probably have to sacrifice a tiny bit of your coral to remove the rest, about a quarter inch into the coral it was growing into, the coral should be able to grow back unless you BUTCHER it, so slow and steady yah know. Get a lightly bristled brush and clean the now sponge free rock as well as you can and keep rinsing with fresh (sponge free) SW.