grey rubber like substance

[Brent RH]

On two different rocks I have a grey like substance. Its too hard to stick your fingernail into. On one of the rocks it has grown onto the substrate in a small space. I removed that part of the substrate and it was bendable like a sturdy piece of rubber. This stuff grows very slow and has spread to about a 4" oval over the past year. (with gravity) One of the rocks is about 11lbs and has over 50 shrooms. I would hate to have to remove the rock from the tank as I am afraid I would kill the shrooms with the labor I know it will take to be removed. Can I get away with continuing to just remove it from the substrate? What is this stuff? Certainly it can't spread fast enough to get out of control throughout the tank? right? Sorry camera not working w/ mac at the moment.

 

[crvz]

Probably just a sponge of some sort, but it's hard to say without pictures for an ID. If it's growing slowly now, I'd expect it to continue at that rate.