What kinda xenia you got? Elongata (brownish) or Pom pom (more pinkish/whitish)?
Be careful what you ask for. That stuff can go like a weed. I had it growing in high flow, medium flow, and low flow. Depending on what sized tank you got, it can take over. I had it growing on my back wall in the return flow and on a snail.
Don't do the bottom of the tank IMO
because it'll reach qwikly to the top and cover everything in its path. Keep it nearer the top if you want other corals to survive. The elongata grows/spreads much faster IME. You might want to talk to your lfs
about trade in as you may begin to harvest it someday. If it's that prolific, you'll have to to avoid it being too heavy/thick, suffocating in its own mucus and crashing.
I harvested mine with small rock rubble, toothpicks, and rubber bands. I'd peel it off rock when thick enuf to sell to the lfs
(at least baby finger thick with 3-4 branches on the stalk), stick the rubber band thru the base, and attach both ends of the rubber band around the rock - avoiding having the rubber on the plant. After 2 days, I could remove the rubberband and tooth pick. Some just toss clippings into a breeder basket with rock pieces and let it do its own thing to attach. One piece seemed to always attach to 2 or 3 rocks instead for me.
Sideways or on top of rock attachments will do, but as I said, keep it near the top of the tank if you want other corals to be in there also.