<i>Calaustrea</i> is pretty notorious for having encrusting live living among the branches. Mine has sponges, algae, 'pods, and several duster worms. Fortunately, none of these are large enough to catch my mantis's attention, but she did recently go to town on two new frags. One was a mushroom with a bivalve attached, which she tried repeatedly to drag behind the LR, and the other was some zoanthids with sabellid tubeworms in them. She broke apart the rock and ate them. If the zoas had been in the way, I doubt it would have stopped her. I did hear reports on this board of more than one mantis going after the pulsing/waving action of Xenia, but both my mantis have ignored it.
Having kept 2 different N. wennera for approx. 1 year in a reef aquarium, I don't see them as a threat to corals generally. They've ignored anything that wasn't small enough to haul off and use in burrow construction, or that didn't have "food" attached. My only other complaint is that they like to burrow and re-arrange things, which is not unexpected, but something to keep in mind with very small corals or corals near the sandbed.
They're not so bad. Compensation needs to be made for them in a reef tank, but it's possible.
