my coparent has our daughter calling his girlfriend’s mom “mimi”

Would love anyone’s take on this.

My coparent has our daughter (3YO) calling his girlfriend’s mom “Mimi”. He sees our daughter about 10 hours a week. He’s been dating his girlfriend for less than a year. Girlfriend and her mom have been around our daughter 5-6 times tops.

I find this to be extremely frustrating. This woman is not her grandmother and barely knows my daughter. I’ve never met her or his girlfriend either. I’m asking myself if I should even allow this to upset me the way it is. It’s not like I can tell him to stop since I guess it’s technically not my business, right? I don’t know. Any advice or encouragement would be nice.