can you break the rock off? is it stable? (the rock that is) if so...twist a screw driver around the base of the coral in the rock. Twist until you make holes deep..Then u can pop it off!! may take some time...but it can b done if u can't remove the rock...
If u can remove the rock...u can cut it off the rock with a water bandsaw..Provided the rock is not too tall....
HTH,
B