Not sure of breed, where did it come from? Do you know how old?
The males will get a curly tail feather... like this...
I have NEVER seen tail feathers on a duck as long as the one yours is sporting, YET...
Another clue that happens in ducks much earlier than the tail curl presents is the raspy voice. Hens will retain a LOUD quack while drakes voice will become more of a soft raspy whisper.
I said possible cross because it looks much slimmer than a Muscovy at this age should, but after seeing the OP's other picture in their other thread I'm gonna say purebred Muscovy.
@Animal-lover339, can you get some all flock crumbles or chick starter crumbles for your cute duck?