Male coworker constantly says "Smile."

I am a quiet person at work. I have a lot of social challenges, like becoming extremely annoyed by pointless questions or feeling crowded if someone lingers too close to me for no productive reason. Because of this I choose to keep to myself, lest I try to relate and overshare, or invite someone into my personal space without intending to by feeding conversation. No one would describe me as shy, but definitely reserved.

There is one older guy (I'm 37f) who has started very frequently telling me "smile!" When he walks by my desk, regardless of whether I am looking at him or interacting with him in any way. This particularly annoys me when he does it as I'm super focused on a specific work task. Not only does it interrupt me but it makes me feel invalidated as a human being.

I have issues regulating my emotions so I don't want to flip out on the guy and get a disciplinary issue for myself at work. I really need this job and I need the team to at least not have a grudge against me. It's a political atmosphere and he has been there a long time.

What are some creative or "appropriate" responses that I can start throwing out there to A) curb his behavior B) not get in ANY trouble or look like an asshole? I have never heard him say this to any other female employees, but I know they would just brush it aside and maybe not have such a visceral emotional reaction to it. Should I learn to just brush it off? Since I don't know how to handle it tactfully, I have just stared at him and remained totally silent (and unsmiling) every time he says it. But sometimes he even says it two or three times in one shift. 🤮

Update: thanks for the replies everyone, I got a good range which is what I was looking for. I'm glad that other people find this obnoxious and disruptive as well, and it's not just my emotional deregulation at play.

Moving forward I will start with "why?" And make him explain his request, and go from there. I feel like that will be uncomfortable enough to stop him from doing it in the future. If it isn't I will have to tell him to stop, but should I do this with someone else present in case my words get twisted?

Like I mentioned I really don't want to rock the boat socially here because I get along very well with everyone else and don't want drama.