If the plug is completely covered by the coral, just use a pair of pliers and snap off the stem and glue or putty to rock.
If not covered then use a razor blade and gently remove coral from plug and glue to rock.
I just snap the stem on the plug then epoxy the rest or crazy glue the top of the plug to the rock. It will encrust the plug when it grows but can still be removed if need be. I do this so the frag does not have to go thru healing and or encrusting again.