My boss's behavior has become really weird; is it harassment or mobbing?

Hey girls, I need to talk about something. At my new workplace, my boss has started messaging me privately saying things like I’m very stylish and all that, but it doesn’t really feel friendly. The other evening when we stayed late at work, he asked if I wanted to grab a coffee, and when I said no, his attitude seriously changed right away. I’m not sure if I should complain or just quit. I’m at the beginning of my career, but honestly, I don’t want to tolerate something like this.