Some sponges might be cooler than the GSP! We have definitely had a couple of occasions where the sponge killed off the coral. One is a GSP rock that is now covered in a funky yellow sponge. Other was a beautfiful zoa rock that was overtaken by this dark purple / black sponge. Someone saw it in my tank (Mizeny maybe?) and commented that she's had problems with that same kind of black sponge killing coral. So yeah, it definitely happens.