Xenia is known to get an infection that causes part or all of it to die. Typically the infected parts turn white and shrivel up, sometimes bits of the xenia fall off. I have experienced this many times with my peach pulsing xenia, but I have enough of it in the tank that I've never done anything about it. However, I do know that the best way to treat it is to carefully cut off all infected parts. But, as stated, your xenia could also be a sign of some bigger problem with your water or lighting.