To me that photo appears to be of a she rather than a he. How old are they? Young Muscovy molt at about 12-14 weeks so it could be possible that (s)he's molting which could make it want to preen a bit more.
Does it have access to fresh water to bathe? Usually ducks that do have few problems with external parasites but that's something to consider.
Just by looking at the photo, (s)he appears to be in good health, the eyes are bright and clear w/o any discharge or foamy eye, the nostrils are clear w/o discharge. The feathers don't appear to be in all that bad of condition considering. The caruncling is nice and bright. Have you held the bird and is (s)he in good condition weight-wise? If you don't already, feeding BOSS helps a lot with feather condition.