Empañar literally means "to fog up" or "to tarnish," like when your breath fogs a window on a cold day.
In this song, Maluma uses it in a very creative and evocative way. He asks his ex, "¿Con quién ahora el espejo empañas?" ("Who do you fog up the mirror with now?"), painting a steamy, visual picture of her with someone new. It's a great example of how a simple verb can be used poetically to create a powerful and memorable image.
This song is all about a guy who is absolutely not over his ex. She's moved to Madrid and is trying to look like she's moved on, but he's not buying it for a second. He calls her out right away: "Según las redes me olvidaste / Pero ese cuento, ¿quién te lo cree?" (According to social media you forgot me / But who believes that story?). He insists the feeling is mutual and that he's been stuck on her ever since she left. He even drops the line "Hasta mi vieja te extraña" (Even my old lady misses you), which is a pretty big deal!
Maluma and Myke Towers spend the whole song trying to convince her (and maybe themselves) that she misses him just as much. They remember their physical connection with some very blunt lines like "Ese culito me extraña" (That little butt misses me). The desperation really comes through when he admits he flew to Madrid for her and even sent her a DM that she didn't even open. He's begging her to come back, "aunque sea una vez" (even if it's just once), because he's going crazy without her.