My mimic tang had the same issue. Pretty much ate anything I dropped into the tank. Pellets, flakes, nori, romaine lettuce, cooked shrimp, so I nevered bothered to take a second look at him. Until one day I was just sitting and watching my tank, not feeding, I noticed how skinny he was. Into the QT he went. At first he was really unhappy about going into the "dog house" and didn't eat the first day, but started gobbling on the second day. So I start Prazi-pro treatment started on day 2. On day 5 of treatment, he stopped eating, and on day 10, he died.
When I told my fish buddy about my mimic, he said that since he was eating, I should have tried medicated food like Ultra Cure PX. I guess that's what I'll try next time...