Good grief, leave them alone! Nobody is coming along and feeding these things in the wild. They manage to get everything they need from the water column. Sometimes nems go through cycles and expell their zooanthellae (sp). I have four RBTA's and two GBTA's in my tank, all come from splits. Every so often one or two will shrivel up and hide behind the rocks and after several weeks come out again looking good as new. Sure it's distressing the first time it happens, but believe me, they wil be just fine.
These are about as rudimetary a creature as you can keep in your tank. They are not as delicate as some would have you believe. Go to any tide pool. At low tide you will see hundreds of them exposed out of the water for hours on end. Somehow they manage to survive. As long as they are still stuck to the rocks and not floating around the tank, they will be fine.