looks like it is a bacterial infection. . how long has he been like this? do you have another tank you can put in? would need to get meds / which ones im not sure. wait for another member to recommend?
do you have a bigger tank/ or even bucket?
not sure how big the fish is, but that may be too small. check your lfs and see what bacterial meds you can get. i think it'll have to be an antibiotic.
i've only had to deal with something like this a long time ago so i dont know what the new stuff is like.
hopefully some other members can chime in and provide some of their experiences
I fortunately haven't had any problems with disease in my fish so any advice on medication are a result of me preparing for the future. So take my advice with a grain of salt.
With that said, frequent water changes are a must and would only help. I wouldn't use the 1 gallon. A 10 gallon or so would be preferable (especially when using meds so that you save money) but if you don't have one available keep him in the 55 gallon. I'd probably do about 30% every other day on tank tank.
As Jay suggested, a 5 gallon bucket that would work. You could also use 10 gallon storage totes. With these smaller containers I'd do daily 50% water changes.
You may want to consider increasing the hiding spots. I used to use black ABS pipe. They actually have a floating kind, a 6 inch section floating in your display tank gives the weaker one a place to hide, away from the fray (main rock base at bottom) and close to food (floating food). I used to put 2 or 3 in each tank of mbuna and they would be used as required by the fish, seemed like someone was always rotating through.