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.
Like Jay said, it looks like a bacterial infection.
Tropical Fish Disease Identification with pictures and cures.
I would try to look for a med that treats for gram-negative bacteria such as nitrofurazone (API Furan 2, Tetra Fungus Guard), Seachem Kanaplex or API Triple Sulfa.
Aquarium Medications Part 2 | Antibiotic & Antimicrobial Treatments
If it keeps getting worse, you could also consider doing a potassium permangenate (Jungle Clear Water. This is not the same as Jungle Water Clear), hydrogen peroxide, or methylene blue swab or dip.
Fish Baths, Dips, Swabs; Including Coral; For Disease, Ammonia, Treatment