Mantis don't directly bother corals at all (other than moving the rocks around).
The problem that I have had, however, is with my more active mantis. they move around a lot, and when they 'run over' mushrooms,polyps etc...(softies), the corals close up and seem a bit traumatized. Again, this is just a problem with soft corals, but I figured that I'd mention it.
-Ron