I wonder if we'll ever get insight on that whole thing. I thought at first that she knew he knew, but that dosnt seem to be the case.
Looking at chapter 1, it
does sorta seem that she's aware that he can't see her expression. She talks about his expressions being great references, but she's not drawing, or anything. And at the end of that chapter, she's reaching toward his desk when he's slouched down falling asleep, saying "I can find something that'll wake you up" - then he startles upward, and when he starts looking in her direction, we the readers get her first face reveal - and she looks super embarrassed, upon which she turns away, Mizuno exclaims "your face was visible just now! Can you do it again?" and she freaks him out with her blank face once more.
That's not proof that she's a Nopperabou, or that she's aware that Mizuno's the only one who can't see her face (or, sees her as a Yokai, depending on how you wanna look at it I guess), but I also think there's something in the fact that she's always facing away from Mizuno whenever her face
does appear.
Maybe it could be argued that she's not wanting him to see her blushing, and the "gimmick" of whether she's a Yokai or not is incidental in those being the moments she "loses composure/focus" and that's why we the reader see her face in that moment - but assuming she's a Yokai, then her turning her face away when it shows up, or it showing up whenever she blushes (since sometimes she's not hiding it; he's just not looking at her, like in this chapter), seems to me to mean that she's aware that he can't see her face,
and knows that embarrassment/being flustered breaks her powers in some way, so she makes sure he can't see her in those moments.
To what end she's hiding her face from him, I cannot say ("she likes him" would make sense if the blushing = breaking power thing is true and she doesn't want him to see her blush
or her face, but that's using assumptions to make assumptions), which is why I think it's still up in the air as to whether the apparent gimmick of the manga is actually the truth, or a giant fakeout that hides the truth of what's going on (Mizuno has some kind form of individual face blindness, or something) - but I guess that's part of the point of the mystery.