Actually, the original wording is correct--or at least more correct.

"popup" is a noun, something that can be closed, so "Close popup" is fine. "Close Pop Up" is not correct, because "Pop Up" is a verb construction: "Pop up that thing, will ya?"

The only thing that we could maybe do here is change "popup" to "pop-up", but we would want to first decide on a standardized form, codify it in the HIG (http://hig.kde.org/), and then change it everywhere, not just here.

It is (also) a noun, though MW spells it as "pop-up" [1]

[1] https://www.merriam-webster.com/dictionary/pop-up

edit: Heh, Nate was faster :)