I used to have the same problem as you when it came to xenia. I had it placed halfway up in my tank and it would just melt away. Finally I got fed up and broke it off the rock it was on and moved the last remaining little frag to my sand bed.
This is what I had left of my xenia as of July 1st 2010
By 07-29-2010 the frag began growing and I also ordered a small frag of what was suppose to be purple xenia but turned out to be regular.
08-22-2010 both frags are growing and beginning to spread.
and as of this month, 11-07-2010 it has grown substantially in the week and a half since this pick, has continued to split grow and walk. So I know where people say it grows like a weed.
So I see a few of your tank info on your signature, but how many lights are in the T5 fixture you use. If it is 6 or more, move the xenia down to the sand bed and let them grow up from there.