Assuming your tank is cycled:
If there are no other fish in the tank, it might feel stressed and that's why it's hiding behind the rock. I suggest some dither fish, like green chromis or some wimpier damsels, like the blue with yellow tails. When they are feeling comfy, other fish feel as though there are no predators around.
How is the tank decorated? If there is only one big rock, it might be gravitating towards it because it's the only structure available.
good luck, but I never trusted fishless cycles and I feel that if your tank is around 55-75 gallons, you will need around 10 damsels to cycle properly