How would you handle a MIL that doesn't respect your wish for no screen time?

I walked in on Grandma holding baby in her lap and showing her something on the phone or playing a game with baby, couldn't quite tell. But the screen was in front of the baby. When I came in the room Grandma quickly moved the phone behind her as if she had been caught. I didn't say anything in the moment because I didn't want to accuse without knowing for sure, but it's really bothering me.

When they first started watching the baby a few months ago I let them know we weren't doing screen time/TV for as long as we could.

If you're also not doing screen time with your baby, what would you do?

No judgement on people who let their babies watch tv, it's just not for us.

Thanks for any advice!