I have an employee at a small IT company that's texting during our Daily Standup meetings. It's only a few texts every day, but they produce visible emotions that can't be mistaken for shop talk.

We're totally fine and open about doing personal stuff at work, and that's how I want us to stay. Their partner works out of another office, so that romance is fine too. What doesn't sit right with me is choosing the only 10 minutes a day when we meet as a team as the time for it.

No one's complained, the work gets done, this distraction just happens when they're very visible and expected to be engaged. I have a feeling that this is slowing down our team cohesion.  

How can I address this, without becoming the kind of boss that butts into everyone's personal lives?

 - I considered discussing group norms without singling anyone out, but if everyone knows, that can be embarrassing. 
 - A "casual" mention can go wrong, if they don't realize how obvious it is and prefer not to bring their romance out. 
 - I can just ask them to lay down the phone, but that's using power, and I want to reduce our power distance, not increase it.

P.S. It might be that these meetings bore some people, and improving them to be exciting for everyone would of course be ideal. That said, we're already great at keeping them short, so I'd prefer that we begin with everyone really being there when we meet.