My xenia touches the A. tenuis and a bunch of other acros, but xenia does not seem to bother them. I read somewhere (maybe in Borneman's book) that a large colony of acropora was surrounded by a forest of xenia as if protected by them. All other corals around them were decimated by some predators but somehow the colony was saved. The author speculated that bad tasting xenia kept the predators away.
I leave my xenia near my SPS's unless they start to shade the SPS's.
Tomoko