they pygmy angels, of which your coral beauty belong, are very cryptic fish. i had a coral beauty in my 75 that used to be up and he was always swimming among the rockwork, lookign for food. he would hang out in the back of the tank for most of the day, coming out very little, if at all! it should just be his normal behavior. does it come out during feeding? try some formula one food, after feeding this mine always came out.
good luck
jon