Husband secretly back in touch with ex girlfriends family

My husband has been a bit more private with his phone than usual. We are very open and tell each other everything however yesterday when we were watching a funny video on his phone, a message popped up from his ex girlfriends brother. I’ve seen it pop up a few times today as well but not said anything because he wouldn’t really expect me to know this name. (We have been together over 5 years and he probably hasn’t seen him in over 8 years or more- he is also a lot younger than my husband so not like there was a great friendship there all these years ago).

I’m pretty disappointed to be honest and don’t understand why there would be any need to speak to him. I worried I was being over the top but when I put myself in his shoes, I would never in a million years reach out and start a conversation with my ex boyfriends brother!

What is everyone’s thoughts on this? I just feel there should be absolutely no ties to an ex whatsoever 😕