It still sounds to me like it melted.
Ive had xenia in my tank for years now and cut tons of frags but before that had a hard time keeping it until I started using reef plus.
When they melt the arms fall off and can look like something ate it.
I have several fish, pistol shrimp, hermits, several kinds of snails including bumble bee, tons of micro stars and never saw anything eat my xenia.
I have 2 kinds the pink pulsing and elongata.
I just cut off the heads and throw them on the bottom on some rubble in low flow to propagate them.
kass