How long has the Xenia been in the tank? I have had anthelia, which has essentially the same lighting needs, grow well for a few months, then slowly shrink as it uses up its energy reserves. As noted above, I expect that mushrooms will be quite happy.
What is the brand of the bulb? I have hunted around for spectra of reptile bulbs, and found that most of the focus is on how much UV they put out. Otherwise, they are considered "full spectrum," which can mean 5000 K, 6500 K, or who knows what else. Because the photosynthetic zooxanthellae in corals don't tend to make use of UV, there's not much point in using a reptile bulb over a reef tank.
I can't think of any reason the lights would be particularly bad for the tank, I would just be surprised if they have enough intensity for anything more demanding than mushrooms.