I am by no means an expert on learned helplessness, but I'm not sure that's what this is. I could see it if the fish was prevented from eating by the other fish, and even when it is no longer being prevented from eating it chooses not to eat and dies because it doesn't know any better. However, this does not seem to be the case. The fish is not prevented from eating when food is dropped into the tank. In fact it is often one of the first to get a chance at food because it is hanging out at the top of the tank, where the food is dropped. Instead it is prevented from assimilating into the territory and rocky habitat, and gets kicked out of the main social hierarchy of the tank. After enough time it stops trying to assimilate and stops eating as well, even though access to food was never the issue.

There are a few ways to try to try and fix such a situation, including quarantine. One example is rearranging the rockscape in the tank to force the rest of the fish to re-establish the territories amongst themselves and take focus off of the weakling for long enough for him to maybe take a spot back.