You can use a Dremel or a wet saw / tile saw / rock saw (diamond blade). If you decide to use a wet saw, be sure to use sw to cool the blade to avoid showering the coral with fw. You'll, of course, want to rinse the saw / blade really well afterwards to get rid of the salt.
If you want something less precise, you could always take a hammer and chisel to it.
In any case, be careful and I'd suggest wearing safety goggles.