Mine hid for about a week when I first got him. he is out in front doing fantastic now. If it is new to your tank I would not worry tooo much (given your water is at a survivable level for an anemone) as he may just be acclimating himself.
When I first got him 4 months ago he didn't come out until he found a spot that he liked. After that he has stayed there and has grown an inch or more. Once he comes out just feed him once or twice a week and it should grow.