GBTA's are a bit finicky as far as anemone's go. I had one that was the size of a basketball shrivel up to the size of a golf ball (later found out it wasn't enough light). That said, if you have a palm sized one, it would shrink to a very very small size, easy to loose, and easy to get sucked up into the return pump
Usually they aren't very boyant and they roll on the sandbed when they are moving, so he's most likely stuck to the backside of a rock. He'll come out in a few days.
-Jeff