Okay so I think this is important. In the manga, Mamoru catches her talking to Luna, but it plays out very differently. It’s more of a question (at least in the translations I’m familiar with), and there’s more doubt.
This, though, seems like he has ZERO doubts about what he heard.
I think he’s already starting to connect Usagi and Sailor Moon. Both have a black cat. Sailor Moon’s black cat talks. He just heard that black cat talk. Who is this girl and what is their connection and why isn’t she giving him answers?
ARE THEY GOING TO FIGURE OUT WHO EACH OTHER IS EARLY ON IN CRYSTAL? Because that’d be interesting.