I've never fed my xenia (and I rarely feed any other of my other corals...every now and then just for fun :mrgreen:).
But, my xenia tend to go through cycles - it will grow like crazy, split a lot, and be fully extended for perhaps months. Then, over the course of about a week it will begin to "shrivel" and end up looking like little purple stumps of death. They will remain like this for up to three weeks or even a month, then return, full-bloom, within a week.
I haven't noticed any particular patterns with water quality (nitrates, pH, alkalinity, etc.), so I'm just accepting it as something that xenia do sometimes (kind of like leathers shedding every now and again). But, they all (right now I have two xenia, but I've had more in the past) do their little shrivel at the same time, and tend to make their comebacks within a day or so of each other. Weird-o if you ask me, but whatcha gonna do?