How (if at all) should I address an employee watching videos/scrolling through social media at work?

For context: I’ve been working in my current role as a library technician and student assistant supervisor at a university library for about a year now; it’s my first role after a few years out of college that’s put me in a position with any type of leadership. I currently supervise 4 hourly student assistants and have supervised a total of 7 over the past year (3 of which I hired after the other students graduated or left to pursue research).

When I first came into this job, my current manager, who was previously in this position before she got promoted to her current role, created a “Student Employee expectations” form that outlined what to do when they need to call out, clocking in and out, got to notify if they’re going to be late, etc.

It also included what they can and can’t do while on the clock, which states they can’t watch videos/movies, use social media, etc while working, but they can listen to music, podcasts, or audiobooks. It also reinforces that students should use their best judgement to avoid causing frequent distractions from work. I really appreciated that this document was already in place because for a lot of these students, this is their first job ever. It really helps avoid misunderstandings between the student assistants and I, so I mostly kept the document as is when I came in. I also go through it with each new student assistant as part of their onboarding. For the most part, almost all students have followed it pretty well and I’ve rarely had issues with any of them.

However, a few months ago, we hired a new student assistant who, for the first few months, I didn’t having this issue with. But about a month ago, he’s started to watch YouTube or Netflix, or scrolls through Reddit on his computer while working. I understand some people work better with background noise, and the last thing I want to do is micromanage.

But I also don’t want to create a situation where other students see it, think it’s okay to do, and now I have to figure out who is or isn’t productive while watching videos or using social media while working. It also seems like it’d be unfair if I let one student do it but not the other, so I prefer to just keep it as a blanket policy that they shouldn’t watch videos or social media while working, but can listen to music, podcasts while working (even just listening to the audio of their shows is fine as long as it’s not visible). They all have to wear headphones for anything with audio anyways because the area where we work is very quiet.

The other thing too is the way our work area is set up: my manager’s workspace is right across to the big table with the computers they use for work, so she and I are both able to see what they’re working on. And frankly, I’m not trying to get called out by my manager for not telling him anything.

(But my bigger issue is that I don’t want create a situation where one student feels it’s unfair they can’t watch videos while others can.)

As for his work, it’s fairly neat, but he works much slower than the other student assistants. How do I approach this (if at all)?