You need a vet. It looks to me, with that definite line below which the foot is dark, there is something wrapped around the leg. At this point, it may be buried in the skin.
I think it would ideally require surgery.
Ducks are very resilient. I would also get the duck on antibiotics.
How old is the duck? Is there something wrapped around the top of the leg, cutting off circulation?
Feel the leg - does the bone feel broken? Is the leg warm or cool?
After checking for something cutting off circulation - if that's not it, and in any case, get the duck into a tub of...