If you can pull that rock out of the tank, put enough saltwater in a bucket to be able to rinse the rock off in. Then use a toothpick to remove all of the little bubbles and rinse the rock very well before putting back in the tank. Another trick that some people use is to cover the bubbles with superglue and then let it dry. You should then be able to pull them off the rock without breaking the bubbles at all.
Yet others may tell you that emerald crabs will eat that stuff, and then you have to decide if you want to take a chance at adding one to your tank or just go about removing the stuff manually.
