No hate pls, random girl on fb?

So there’s this girl we will call her Michelle. Michelle and my SO having been liking each other’s post/ commenting (every post) so I was like hmm..never heard of her. So I asked my SO who she was, and he says “I don’t know some girl I went to high school with” so I told him I just didn’t feel comfortable with them being friends on Facebook & to please delete her. He gets mad, tells me he doesn’t need to delete anyone. I’m like okay, weird. So I go on his Facebook (I know, bad. Please don’t judge) and I delete her. Two days later...added again. I was like WEIRD. so they’ve been friends on facebook for about 2 weeks and they’re doing the same shit. Comments and liking post. So I go on his Facebook AGAIN and delete her. 5 days later. Added again. Like what the actual fuck? Obviously there’s something going on there & if I ask him he’s just going to blow up. I don’t know what to do.

Edit- I’m not crazy. He’s cheated in the past and has lied to me multiple times. So I don’t trust him. I don’t care that he has friends of the other gender but when he use to lay in bed with me and flirt with other girls (send pictures & askfor pictures of the other girl) Yeah, excuse me for being a little sketched out of a “friend” I never heard of. I don’t monitor him, I’m friends with the girl on Facebook too. He’s added about 100+ girls within the month. I only know because it comes up in my newsfeed. He use to not use social media so that’s why this is all so sketchy to me.