Without pics of the turtle it would be hard to tell what kind of illness it has or if it is even an illness. With that said, turtles do shed their skin, just like all reptiles do. Now if the neck looks red or irritated than you might have a problem. Your best bet is to treat the tank with acroflavin, it will dye your tank green for a few days but will kill all the bacteria that is in there without hurting the turtle or the fish if you have some of them in there. If you still see problems persisting than I would remove the pleco. Also another thing you can do is dry the turtle off and use some topical antibiotic (neosporine) on the neck, allow for drying and put the turtle back in the water, repeat this process until you no longer have the skin problem. I hope this helps you out a little