If the bits of poop are not blocking the vent, it should be fine to just leave them.
I think that is strange, but I don't think there's anything you can really do about it. Either it will be fine or it will not be fine.
Since it was by itself, there may be something wrong with it-- either the parent may have rejected it, or it may have been too weak to keep up with the rest. Or something could have happened to the parent, in which case the duckling itself might be fine.
If you're fairly sure it's wild instead of domestic, I suggest you contact a local wildlife rescue place and ask for their advice. Wild ducks don't usually make good pets, and any ducks do better if they are raised with other ducks. A wildlife rescue might have other ducklings to raise it with, and should know if there are any things specific to wild ducklings that people with domestic ducks don't usually think about.