is the rock removable? or is it buried/too big?
if on a small rock...take the entire rock
if on a larger rock or a rock you want to keep that is still removable...remove the rock, cut the colt, replace the rock.
if the rock is not removable , start a small siphon near the coral to pull out anything the coral releases in defense (also...run carbon if you can) and then cut the coral off the rock (aka frag it)