I'm fine with working, I have my own pace for doing work.

For context I'm a part time custodian working at a university, 4hrs shifts during the week from 8am to noon. My duties aren't bad, just dust mopping, wet mopping, cleaning and restocking the two bathrooms, emptying trash cans, sanitizing tables, cleaning the breakoom, etc.

Even when I got as slow as I can to maximize time, I still finish my tasks early with nothing else to do. So I'll browse my phone in the janitors closet and occasionally go out and make sure everything is still clean. I've been caught by a coworker 2 times already for being on my phone. "You'll be written up if they see you. Try to always look busy" and how do you expect me to with no other work that needs to be done in my area?

I just don't understand the whole "Look busy/like you're working all the time" thing.