I'd check pH first. Practically every problem i've heard from people with Xenia has been a pH issue. When the pH is below 8.1 for long periods, the colony will look flaccid and unhealthy and the pulsing action slows down and becomes weaker. Also the tips of the tentacles curl outward. Also be aware it's not uncommon for colonies of Xenia to suddenly 'crash' for no apparent reason, after lots of strong growth. To avoid this, keep a check on your iodine and iron levels. A good trace element booster will help with this. Lastly, make sure nothing in the tank is picking at it!
Are you treating your tank with iodine? Here's what I use weekly to keeps 'em pulsing strong:
Kent Lugol's Solution provides a strong source of free iodine and iodide to reef inhabitants and macroalgae. Although it’s very effective when used with stony corals, systems containing xenia and soft corals are particularly benefited by the use of Lugol’s Solution. Used as a dip for stony and soft or leather corals, Lugol’s will help rid the animals of unwanted parasites and harmful bacteria. This product helps foster improved coloration and prevents bleaching of all corals due to changes in light intensity, and enhances coral polyp expansion. Kent Lugol's Solution provides a key nutrient for marine invertebrates and fishes, and contains no phosphate, nitrate, gluconate, or other sugars.
