That is odd as far as the xenia goes. I would be very doubtful that a mithrax (either red or emerald) would eat xenia as it can put off some pretty nasty toxins. I would check your other water parameters as soon as you can. Don't want to ask a dumb question but are you sure that the xenia isn't just closed up all the way? If you do think something killed off your xenias I would replace your chemical filtrant (carbon, chemi pure, etc) and if you have a protein skimmer start skimming a little wetter than normal to prevent a buildup of toxins in the tank.

here is a little more info on that red crab of yours, (from sea life inc.)
Red Mithrax crabs, Mithrax ruber, are very close relatives to the green emerald crabs, Mithrax sculptus . The red mithrax crabs tend to be more of an offshore, reef dwelling crab, so I think they are actually more suited for a reef tank than the green crabs, which tend to be more of an inshore, non-reef dwelling crab. Both crabs will clean bubble algae, and hair algae, and both will clean other unwanted fleshy macro algae from your tank. You can mix them freely with the emerald crabs and they will get along fine. Maximum size is about 1 ½"" carapace width, and we prefer to ship them at about ½"" to ¾"".