Anytime you cut any coral it is more prone to illness. Most of the time fragging xenia will cause it to shrivel up a bit. Durining this period I would add a little extra iodine to help pervent illness as well as to provide the xenia with extra food. I would also consider moving the xenia to another tank. when you cut the xenia it does release a toxin that can be harmful to other corals. If you don't do this at least, toss a little extra carbon in your tank, to eleminate any toxins in the water. To be honest there is a little easyier way to frag xenia. just put rubble rocks around the main colleny. It will spread over time and just slowly move the rock that you placed away from mother plant over the corse of a week. You won't run into the same problem that you are having now and it is overall etter for the tank.