The emerald crabs aren't going to bother the coral. I'm talking more of the ones you may not know you have. The joys of live rock...LOL. The photo really doesn't indicate a crab though. The xenia doesn't look chopped up...just curled, but still doesn't hurt to look.
Definitely test the calcium especially since you have been adding it to the system. Calcium level in a reef should be 450 (no higher). Too much calcium can cause problems.
There are hairy mushrooms that will make good ideal borders for the xenia. The xenia doesn't seem to cross them. Though the mushrooms aren't gonna jump and attack anything. Just don't have them touch other coral. Hairy mushrooms do sting.
You could still try to put a small dose of iodine to see if the xenia does perk up. What is in the trace elements may or may not be enough. Won't know until you try. See if your LFS
would give you a little bit instead of buying a whole bottle. Some places might if they use it themselves and have a store use bottle opened. Doesn't hurt to ask.
Do you run carbon in the filter? If you do, take it out. Reef systems should not have carbon 24/7. It can rob the system of trace elements. It's better to run reef grade carbon about a week once a month if you use carbon at all.
How long have you had the xenia?
We, as a people, know so much more about outer space than we do about our own oceans. This lack of knowledge can very well spell the dangers that lay in wait for us.
The oceans surely would swallow us before a rock comes down to smite the planet of it's life.