Oh, I see. The ventral side sometimes can have strange colorations, so I wasn't sure if that on your pleco was the norm or the suspected issue.
I must add I've never dealt with such issues. Is it really skin? Not the intestinal / internal organ issue? All my experience with skin problems (infections?) has to do with inflammation = redness, lesions, open sores, ulcers, etc. My impression is has most usually or even only to do with external parasites.
Discoloration you are pointing at, may root in the internal issues, as the skin is semi-transparent and thin and the color of the tissues, organs, etc. underlying the skin affects overall color.