I just put a new fairy wrasse into my tank. Saw him in the corner for a few hours and now's vanished and not appeared for over a day even during feedings...is he a gonner or hiding? Is he one of the varieties that burrows in the sand?
Likely just hiding. Fairy wrasses (if it's a Cirrhilabrus) will not burrow in the sand. They will, however, tuck themselves into tight spots in the rock and create a mucus-like cocoon around it at night. They acclimate slowly and often take over a week to emerge. Your wrasse is probably just hiding somewhere in the rocks.
I put a Cirrhilabrus in my tank, watched it swim into the rocks, and didn't see it again for nearly a month. Then one day I was sitting on the couch watching the tank and a fish I didn't recogonize darted out. He seemed totally healthy and happy. It was really wierd.
Lucky this time .... make sure he can't jump out of the tank anywhere. I had a small lunar wrasse jump out a tiny opening in the back of its 55 gal tank one night and found him (somewhat crispy) under the tank in the morning! :?