What an adorable face! Yes, he is quite thin. Also, in the pic his fins appear completely wasted. Not frayed or chewed, but wasted. The spikes are there, but the thin filmy part of the fins looks gone. Is that the case?
My guess would be malnutrition. How long have you had this fish? Have you noticed him being harassed or scooting away when other fish approach? How are you feeding the fish? These fish can not chew, thus they must be able to gulp bite-sized morsels all at once, and swallow whole. They are omnivores, so I would add some vegetable material to that diet. I have seen bottom fish who appear to have ravenous appetites starve to death. Competition for food with more aggressive fish who have first opportunity to eat usually leave little for the little guys on the bottom.
Try to add more food variety to the diet. I know you said you were feeding mysis, plankton, brine shrimps, which is a good diet but what exactly is the product you are using? Frozen, freeze-dried, flake? Soak the food in Zoe or Zoecon for 1 hr in the fridge, then, offer the food directly to this particular fish in a syringe [most pet shops have these as they use them to feed baby birds, etc]. Give your other fish their food, then gently get the syringe with food down in the vicinity of the blenny [wo frightening him] and lightly squirt out bits at a time for him to eat. Do this 3 times a day. He may be frightened at first, but he will soon realize that what you are doing is a food source for him, so he’ll get used to it. Be sure that you hands/arm are very clean…including free of soap!!
See if this helps.
I don't get it. It happened so fast. I am wary of buying another one if he dies. I sure hope he pulls through.