In my expierence xenia sp is one of the few softies that "glue" really well.
I say this because given the same water conditions that allowed it to spread in the first place will also allow it to attach to the rock (rubble or whatever you glue it to) in a matter of just a few days instead of weeks or months like many other softies.
This is especially true with xenia that is growing on the glass because it has already developed a hard leather like base that makes attaching it with a gel superglue a snap since the base is already formed you do not have to worry about the frag expanding and popping the whole piece off the rock.
Other softies like colts, tree leathers and even toadstools produce much more slime than most xenia when cut. As a result using any one or a combination of glue, netting and rubberbands is much more practial and productive to secure the frags with.
SiF