my current boyfriend is playing Yahtzee on his phone with an ex girlfriend. It's not just once a day. Anytime a notification comes in he picks up his phone to play. They don't chat, it's just playing. Should I worry that they have this "thing" ? Should I ask how he would feel if I was doing that with an ex of mine?
I think it's ok to be friends and be in contact with exes, unless they were abusive or something like that. So it would not bother me. Now if a guy had an ex as his best friend and they hung out all the time, that would be worrisome, phone games are pretty harmless.
I believe you confront him about it nicely and casually and just say "insert name here... I just wanted to let you know that playing the gamr with so ans so is kinda hurting my feelings and here's why" then you can express your feelings to him. After all, a relationship is about communication.