Pink Sponge?

[krissyfish20]

I've noticed in the last 3 weeks or so this growth on my watermelon mushroom rock that looks an awful lot like a sponge. It's pink, and I'm just wondering where it came from and if it is okay for my tank. I know sponges are fine for tanks (you can buy them at the store, right?) but I was just wondering if it might be something other than a sponge that could harm my tank. It's not cyano or anything, and my mushrooms dont seem to be dying. My tanks is 3 years old and I havent introduced new live rock in there for a long time so I dont know how it propagated. Has anyone else ever had a sponge randomly grow in their tank starting from nothing?? I don't have a picture of it but all I can say is it looks like a sponge.