Hiya, and welcome to BYC!
This one not growing, could either be a FTT (failure to thrive) which I don't think it is, or just a runt. Runts grow up eventually and may be smaller than the rest.
Baby chicks can engorge themselves to the point of it looking like it's something wrong. I'd tend to think this is what's going on with a chick so young.
Housing chicks with ducks can lead to problems, such as is there clean water for the chicks? Are these chicks free-ranging with mom? Assuming so, so they would be getting grit.
I would try making them a shallow dish of wet crumbles and give the ducks their own dish as they'll love it too.
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