Happened to me while talking to a teacher. The spit came from him. He awkwardly reached out to wipe it away, thought better of it (probably because it was already absorbed by my shirt) and visibly shrunk a few centimeters from embarrassment. All while our discussion went on normally. But the power dynamic was switched around in one instance.
I’m pretty big on this kind of thing… spit lands on my arm, I continue the conversation normally and after a second’s delay slowly move to wipe my arm off without mentioning it.