I was just thinking it makes sense if Fury came up with the name. If nothing else, as a codename for his files to protect her civilian ID. It would make sense since he knows Carol is a Captain and knows about Mar-Vell. I imagine Carol being okay with it once Fury told her that was the codename he used for her.
Or, I can also imagine Carol coming up with it in a conversation with Monica if she asked Carol what her superhero name was. It'd be interesting if we find out Monica came up with the name.