I dont know what i should do..please advice.

This all started from March.My husband found his second cousin on Facebook.She is 28 years old.He wanted to use my phone and he left his facebook messenger signed in so everytime she replied it kept popping up and i thought lets check out what they are talking about.It was midnight and he was working late.I saw how he was talking in a flirtish way and she jusr answered like bro what is wrong with you why are you talking like this and stuff.When he came instead of asking him nicely I got mad at him and started raising my voice which i haven't done before.I mean once in awhile but not this way.He kept calm and he was smirking and then he says "I cant believe you are acting so stupid i didnt expect you to be this way i thought you were much smarter!" He says how its how he talks to all his girl cousins it doesnt mean anything.The next day he brings up tje topic again and says you know i could make a different facebook account and hide everything from you.He says he told his mom and his mom laughs and says that you arent always suppose to share everything with womens they always take it too far and im Like Are you serious now?Wth!?

Today i was on his phone looking up a recipe because my phone was charging. I didnt even have the intention to look through but when i opened the browser i see he made an account on facebook with differenr name and talking to her like before.I saw he started talking to her right after i found out.He calls her and talks to her in his car instead of the house.I dont want to make it huge deal now because then he will make another account.I explained to him before and he wont say anything besides its just a joke nothing serious.As far as i see she doesnt care about the way he talks she just says hey look i have work to do or like i need to sleep she doesnt even go along with what he says she ignores it and says oh or like hmm.

He calls her cutie pie and says how much he misses her and nothing feels good if she doesnt talk to him.

I think im just overreacting.

Any advice suggestion is appreciated.TIA