Kalk or Kalkwasser is a concentrated solution of calcium hydroxide which is used for calcium and alkalinity maintenance; you'll see it used by a lot of reefers trying to raise their cal/alk numbers. I believe that Aiptasia X is essentially a strong kalk solution. You can buy kalk in most LFSs and make up your own paste like solution which you can squirt through a syringe or just use Aiptasia X; turn off your pumps; scrape as much of the Xenia off your rocks as you can, and then squirt the kalk on what's left of the Xenia. Aiptasia X out of the bottle may be too weak to kill the Xenia the first time and so you may want to hit it a few more times. If you buy the dry kalk you can make up a paste that is very strong.
Let us know how it works. BTW, some of us, me included, have a soft spot for Xenia and wish you weren't so far away across the pond so we could rescue it, ha ha.
Good luck