It's the coccidiostat that's often put in chick crumbs, ie amprolium.
I do prefer chick crumbs with it in for chicks, as I did have some quite ill with coccidiosis once.
However, I also have experienced ducklings fed it and they were really poorly. Once we realised that's what was being fed, and advised on changing to an ACS-free crumb, some of the ducklings recovered, but unfortunately some were too far gone and died.
Droopy wings could be angel wing, which can be caused by feed too high in protein, so that the big wing primaries develop too fast and before the wing muscles are capable of holding the weight. It's often an issue in geese, so I can see it could be in Muscovies too. The ducklings I saw that were fed the wrong chick crumbs were just small, weedy and sick. (The treatment for angel wing is to bandage the wings to the body, while the bird grows and 'catches up')
Most layers pellets done have ACS in them, as by that age, pullets have developed their natural resistance - but you know what they say, always read the label.