The percula sounds like burns from it hosting something in your tank. I can't see your zoos stinging it though. Either way I wouldn't worry.
As for the sebaes that sucks, sounds like ICH. If you can try and get a cleaner shrimp ASAP
. Also give it a FW
dip if you can. Sorry to hear they looked really healthy when I picked them up, and im sure you can agree.