Thinking...
While worming can be necessary, it also seems like weeks is not the standard treatment period for worming.
Sometimes the strange green poop indicates internal organ stress. Sometimes toxicity.
I would stop the worming for now, I would put a teaspoon of food grade activated charcoal supplement in a pint of water for drinking overnight. That absorbs toxins, I am told.
The things I can think of that would cause leg issues are niacin or general B vitamin deficiency, sprain, spinal injury, rarely a tumor.
Does he use the leg at all, floating in lukewarm water? Don't leave him unattended, by the way, in the water.
Regarding moistened food, with great respect for subhanalah, I would not panic about it, and I would not leave it available for more than five hours or so, because it can get moldy - depends on how warm and humid it is in the brooder, and just the spores in the environment.
Fermented feed is a good idea from anything I have read or seen.
I would shift from the wormer and antimicrobials to restoring gut flora (there are poultry probiotic products, and you may be able to find a dosage recommendation for human probiotic products also) for now. Don't want to overload the system, but build it up.
I understand why you have been doing what you are doing. I am also trying to step back and look at the whole picture.
I just wonder if he could have gotten his leg trapped, and hyperextended while you were away. Ducklings can get into preposterous trouble.
I would make Epsom salt compresses and wrap the leg (while holding him gently) for ten minutes three times a day. Pulls can take days and days to improve.
Don't let him ingest Epsom salt, it is a laxative and that baby does not need any more tummy challenges.
