It cou7ld be getting ready to grow anouther head or just exstending it mantal or what ever it is called. trumpet corals will exstend it mantal and grow a new head. Sponges come in a wide veriety of colors, shapes and sizes; if it is a spunge it should have what loos like 2 mouths one is the in take for filtering the water and the other is the output for the nice clean water. If the other spunge wasn't there I would definitly say the coral was propigating. I have trumpet coral that has just done this, so it is a posibility. Just leave it in the water and watch it closly and you will either see the bony structure start to build up a bit or the flesh spred. If the flesh spreds alot then you may want to do some thing or seek out expert advice on the matter.

In the meen time you may wat to have that spunge turned toward the light that will take care of it's self.I say this because the spunge will over grow the coral I have seen spunges fill an entire cavity that I could fit my hand into.

Bill