He just realized she's paying special attention to him. Ignoring her creepy look when she's not stepford smiling, she's a girl giving him special attention.
I assume that the joke is that the girl is shy and has a crush on him, so the only thing she can do in front of him is smile, but since he's a bit of a scaredy cat (and not a very bright one), she looks to him like a creepy killer in a survival horror game.
... or maybe he's right, she's a sociopath and he's the only one to notice. Whichever.
An ending for this: They start getting along then she invites him to her home, and on the last panels she closes the door behind them and suddenly switches to her other expression while looking at his back. Gotta keep it ambigious.
she's his daughter from the future who traveled back in time after his future self walked out of her and her mother's life, citing, "Going to the store for cigarettes".
Seeing how miserable her mother becomes after her father's disappearance, FMC vows to change the course of history, even at the cost of her own existence.
That's why she pays attention to him so much, with that constant unsettling smile hiding something beneath it,
because she can't help but be reminded of the person he becomes in her original timeline.