I would bet almost anything that it has to do with the fact that he's crested. The crest on ducks is actually caused by a skull deformity, and sometimes there will be bits of fat and bone that protrude up into the crest, but you would never see them because they are inside the crest. Sometimes their skull doesn't even cover the part of their head where the crest is... it's just all fat and skin and feathers. I bet that little claw is just a random piece of bone sticking off his skull!
I think he'll probably be just fine, but once in a while crested ducks have neurological problems like poor balance, so you may want to keep an extra eye on him just in case.