How big is it? How many other fish are in the tank, and what kind of fish?
IME, Foxface Rabbitfish are very skiddish. They are spooked extremely easily. In nature, they are used to being in small shoals I believe (unless you have a Magnificent Foxface, as these only form pairs in the wild, IIRC), so it takes them a while to get truly comfortable in a tank. The larger/older they are, the longer it can take.
Larger, fast swimming fish like tangs and angels can also contribute to it hiding vs. a tank with smaller and more docile fish. However if it is the only fish in the tank, I would expect it to take a long time to get comfortable, so hopefully this is not the case.
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