Tough spot
Metal is dangerous for ducks to eat. Did the beer cap go all the way down the throat? One concern is choking. You saw the duck eat it? Little Muscovy - adult or duckling?
A good vet would be best. Sometimes they pass things like this that they eat, sometimes they get stuck.
There are medicines called chelating substances, that can help prevent metal from getting into the bloodstream - really, a knowledgeable vet is needed.
@casportpony
@Orca5094
I saw one web page that said the caps are generally made from steel. That may be good news, as steel is not a very reactive metal. So the biggest concern is that the cap might cause a digestive blockage. Does the duck get grit?
There is a possibility that the cap will pass and she will be okay. I would watch her closely, pay attention to her poop - if she gets blocked, there won't be much coming out.