You can give ducks vitamin b complex.
@HollowOfWisps mentioned it the post above. You can also get it from TSC as well. It says injectable, but you can just take give it orally. You can give the duck 1ml of the vitamin b complex a day. If it is a naicin deficiency, you should improvement in the way it walks. However it can sometimes take multiple weeks to see the effect.
For the eye, the best things really are saline flushes, baths in clean water, and wiping the build up with a damp washcloth. The terramycin can clear any infection up, but it will not clean off the buildup around the eye. Only proper cleaning will. One of my Pekins had a minor eye irritation, and I believe it took a couple weeks to clean all the buildup off. If you are still concerned, a vet can provide you a more expert opinion.