Figured I'd give you a bit of an update. I removed the crabs and about a week later I noticed that the yellow tang had ich
I removed him and put him in my quarantine tank. Still noticed the emeralds picking at him so I gave the emeralds to a friend. Treated the tang for ich and after about 4 weeks he was good as new. all healed up and everything. I have had him in my display tank for the last 3 weeks and he is doing excellent. I talked to the guy at my LFS
and he said what was happening was that my tang was itchy ( ich) and was probably looking for the crabs to clean off his parasites. Not knowing that the oportunist emeralds were going to try to eat him, he went to them. He told me all of this before I even told him my tang had ich. Just figured I'd let you guys know in case this strange sight happens to someone else. Thanks for all your help.