Looks like Xenia, possibly the pompom xenia, and yes, that stuff will just about take over a tank once it gets going.
To kill it, hopefully you are able to remove the rock with it on it, through it in a fresh water bucket and leave it for a few minutes, that will kill it off. If you can't, then kawlk paste, or blowing boiling water on it, if left in the tank on the rock, its a bit tougher to kill it all off.