I don't want him to be friends with ex.

[Getoffmyhat]

My partner recently got in touch with an ex girlfriend. They are fb friends and he wanted to reminisce about a funny time they had once together. Anyway, I'm not 100% ok with this but partner has been transparent and it was nice he was trying to include me.
Ex girlfriend is now messaging more often and now signs off with kisses. The odd code word from their relationship or phrases that aren't funny to me (because I wasnt there!) are starting to creep in and I don't like it!
I know it makes me sound jealous but I worried they will develop feelings for each other again.
They split up 20 ish years ago when she went off to uni.
Would ibu to ask him to give the messaging a break for a while? Just until we can establish some boundaries? I dunno if I sound nuts here!

OP, I too would be very uncomfortable with this.

Your H may be showing you the messages, but nevertheless he is crossing boundaries with his Ex. They are developing intimacy right in front of you, and things are escalating with their now daily contact and her addition of kisses. He is enjoying the ego boost and is failing to draw a line. Her recent early morning messaging and encouraging him to meet up for Live Action Role Play are evidence that things are ramping up.

It is time for you to set your own boundary. I would have a serious conversation using the wording that @YoniAndGuy suggested on page 2. He needs to understand that he is jeopardizing your relationship and has much to lose.