According to Marine Center: "It will usually remain buried for a day or two when first introduced to the aquarium. It can be housed with a wide-range of fish species, although be sure not to house it with any fish that may pick on it, particularly initially as Halichoeres sp cannot tolerate any harassment, particularly initially. Can do great in the right environment."
Maybe the tank rearrangement scared him into the sand. I hope you see him soon though! They are lovely fish. Also, if you could let us know if he is reef safe!
What kind of a cleanup crew do you have in the tank?