Well I have no hands on experience with these guys, I can try to help some.
The "pom poms" these crabs have are actually anemones (small ones). I believe in the wild they don't use the same ones for ever and actually switch them out.
Now it is possible that during a molt the anemone got away and he hasn't found it yet and may eventually get it back. You might want to search around the tank to see if you can find a small anemone somewhere hiding (probably like finding a needle in a hay stack, LOL).
It is also possible that the one anemone was dying (or is dead) so the crab let it go.
I assume we are talking about one of these guys:
"Simply put, you believe that things or people make you unhappy, but this is not accurate. You make yourself unhappy." Wayne Dyer