Like Ping said, Xenia does take time to acclimate. When I trim my Xenia and Anthelia in my 12g and put it in my 30g to mount it, it takes a few days (sometimes a week) before it really opens up and starts pulsing. From my experience when Xenia is dieing it shrivels up to next than nothing and disappears within a day or so.

They don't need much light to stay alive, of course more light is better. Example: I have a 20g with a standard 20w fluorescent bulb that I keep all my LR rubble and a few medium size pieces of LR in. The only reason I have it is to keep the rock cured for my next tank. Well, one of the pieces of LR had a stalk of Xenia on it and it has managed to stay alive for 5 or 6 months under that lighting.