She either has a deficiency in niacin or has some sort of muscular-skeletal deformity like varus-valgus. I know you said you already checked her feet, but could you do another thorough exam, specifically looking around her hock area for lesions. Birds with leg conditions like this tend to apply great pressure to that area which can lead to a bone infection.
I think this duck would benefit from a daily niacin supplement, water therapy often, possibly in Epsom salts to reduce swelling and inflammation. Keeping her on soft padded grounds to help relieve the weight off her legs would be good. Adding flax seeds to her diet, or any other omega-three source can help prevent the onset of arthritis, and help reduce any pain.
Regarding the poop color. Bright green poop generally means there is some sort of infection going on, but the past few months I've talked to other members whose duck was pooping almost an identical color, and there were no problems. It would still be good to check her legs, for any sores, as if she does have an infection, that would be the most likely entrance into her body due to her condtion.
