Okay, I looked back at the raws for these two chapters, and they're actually referring to different things.
In chapter 3, Miu says that whenever Matsuri looks behind her, she looks over her right shoulder too often.
The most common reference I can find to this is a belief that says if a person looks over their right shoulder when you call their name, they see you in a sexual/romantic way.
If they look over their left shoulder, they just see you as a regular person.
I'm not sure how this is applicable to white hair, though. The closest I can figure is that it's just a mild general insult to say someone looks over their right shoulder too much when they look back.
In chapter 51, the sign on her back literally says, "Turning (as around a corner) to the right."
I don't know if it has any cultural significance, but it looks like this is just similar to a generic "Kick me" sign.
Like a car that forgets to turn its blinker off, Matsuri is perpetually walking around with a sign on her back that says, "I'm about to turn right."
It's funny because then she actually does turn right.