I FOUND IT!!!!!!!!!! YAY!!!!!!!!!!!!
the pistol shrimp caused a major rock to roll last night (undermined it enough) and when it did, it opened up a "cave"....thus....I found where my Xenia had been swept off to....
I put it back on a rock in the morning and it seems to be "happy" for now.....though the "arms" are really really thin.......
so where do these guys like to be? do they like lots of light? how about flow?