As rudyistaken mentioned, G. ternatensis is very given to 'fragging'. Dr. Roy has mentioned that this species lives in live coral heads, and mine seems to like to whack away like she was in a fragging contest.
I have twice put small rocks with zoos in her tank, both times things ending with her 'kidnapping' the little rocks, to use as decoration somewhere. She is extremely aggressive, and trying to find them in her den would be suicide (At least for my fingers).
This brings up the only real danger for corals, the chance that the mantis might decide to show disatisfaction with your aquascaping projects. They tend to move around the rocks, with or without attached corals. I have random mushrooms in with 5 or 6 of mine, and the rocks were moved around quite a lot (And still are in a couple of cases).
The little guys don't bother much in the way of corals, at least not from my experiences, and both of my O. scyllarus ignore what little coral life is in their tanks (Low light mushrooms,a s strong lights are bad for O.s...).
-Ron