Looks like an air pocket. Nothing to really worry about IMO. If it bothers you, take the rock out of the tank for a few (2-3) minutes and he will deflate. Place the rock right back into the tank and everything will be fine. Barry
ITS NOT A TUMOR!!!
Seriously, get a small needle and pop it. I can't see how this would hurt it, you can cut them up to frag them you should be able to pop it with a needle. Or just make a thin slice on it and let it deflate.