The Wikipedia articles for "Set phrase" [0] and "Compound (linguistics)" [1] actually don't offer much of a distinction, so it's hard to say which of those is correct. Regardless, a compound with spaces ("open," as you said) is multiple words, not one word.

[0] https://en.m.wikipedia.org/wiki/Set_phrase

[1] https://en.m.wikipedia.org/wiki/Compound_(linguistics)