The feather dusters you are talking about are not the big kind, they're a Feather Duster Cluster? I had a cluster of them that really didn't do well. Sometimes they blow right out of the cluster and float around the tank. They generally settle on a rock somewhere. Was the one on the sand still alive? If so just pick him up and put him with the others. I didn't know that emerald crabs bothered them at all. We have 6 emeralds in our tank, and they never bothered them. It sounds like you really need the emerald to work on that bubble algae. Good luck!