In this movie clip, there's this conversation between Monkey and Beetle, starting at 50 seconds into the clip:
Monkey: What?
Beetle: Well, fast learner. Did you know you could do that?
Monkey: Show off.
The context is that while Monkey and Beetle were arguing, Kubo made a boat.
What did Monkey mean by "Show off"?
If interpreted as an imperative mood, it doesn't really make sense to me.
If it's not, what is it?
show-off - a person who tries to impress other people with his or her abilities or possessions [Merriam-Webster]
Typical usage here is for humor. It invites the audience to interpret Monkey has saying, "yes, you made a boat, and it's very good and I'm jealous that you could do it so fast and you know it, so I'm just going to call you a name".
Think of it like this:
Monkey: What?
Beetle: Well, fast learner. Did you know you could do that?
Monkey: (You/they are a) show off.
