Since you said that you've seen it "many times", and you've picked it up..you've fed it-so you know that it is eating, which all leads me to believe it is doing just fine. You may feel sorry for it because of the leg but it appears to me there is no real need to interfere. If you believe the leg could be tucked up then call animal control, inform & let them assess the situation. You can let them know you are willing and capable of taking in the duck if need be but let them decide if that is necessary. As you've said, we may mean well but our interference causes more stress or harm than good.
Good luck and keep us updated, if you can.