What do you do about subtle harassment?

These things happen, I'm willing to bet, to most if not all professional women. Subtle harassment. Stepping into your personal space. Leaning in too close. Microagressions. Etc.
If it's a person that I see/work with repeatedly and they've thrown these red flags out before, I have my strategies to stop repeated offenses -- put up my professional barriers, drop my typical friendly kindness so as not to "invite" unwelcome and unprofessional intrusions into my space, etc. (Also with these repeated things, please share what you do that works for you, but the main question...)
What can I do in that first instance? I was at a graduation yesterday, in my freaking regalia -- not a sexy sight -- and was introduced to an older male colleague. Now, he's a colleague in the sense of co-professional at this university, but in three years this is the first time we've crossed paths, and I doubt we'll cross paths often (though if/when we do, I will definitely be putting up my barriers as mentioned before).
We're introduced, we shake hands and say the ordinary how do you do nice to meet you things. That's the whole interaction.
But. BUT. He does that creepy gross thing. It's so subtle that I'm not even sure the other folks in our conversation could have seen it. He's shaking my hand but he's holding my hand... different. Like a caress. Holding it too long, too intimate. He's giving me this look (eye contact, mind you, he's not looking me up and down) that is stripping me of my colleague/professional status and instead putting me in the "you are an attractive female body to me" status. Like he's approached me at a bar rather than is being introduced to me by fellow professionals at a professional function.
What do you do in that situation? What could I have done? What can I prepare myself to do next time someone catches me off guard with first time, subtle, diminishing, creepy behavior? It seems like it's so subtle that calling it out would be overkill, would make me the bad guy in the moment. I know how to shut it down from happening a second, third, or fourth time; I've figured out the body language and voice tones to use when I know ahead of time that this guy is a creep. But what do you do when it's the first time? How do I shut it down in that moment? How do I take control of that first moment? Have any of you figured out a good strategy?