I know people who've got leathers (like toadstools) growing out of their tank...the part that grows out of the water sort of just curves back into the coral so that its at least barely under, and that particular part never really looks that great....doesnt look like its dieing or anything, just never opens, you know?
But if you're talkin about stuff like xenia, zoos, shrooms...etc... I wouldnt even worry. They wont be able to really stand up if they're out of water, so they wont grow out of it.