If you can, pull the rock out of the tank and then just slice a piece of the m'room off (assuming you want to keep the bulk of it where it is now, otherwise cut most of it off and where you leave a little attached to the big rock, a whole one will grow back). Take the cut piece and put it in a little plastic cup with some large gravel in the bottom, cover the cup with some netting (a piece of bridal veil lace works very well) and rubber band it around the top of the cup. Set the cup in the tank and after a few days, the mushroom will grab onto some of the rubble. Then, glue the rubble to the new rock in the reef.