It's raining teeth again, thanks pollution! I ran through the streets with my guitar in its case and over my head, the little white pearls bouncing off it and falling to the ground. Every now and then, I would see a monster mouse appear, grab a tooth in each of its mouths and then scurry away.
The concept is interesting, but I feel the prose could be improved. Lakey suggested exactly the thing I was going say about the the first sentence, as well as commenting on that tense shift.
Actually, most of what Lakey said is some variation of what I was going to say, so... there's not much for me to say on that front (without going into too-specific suggestions)
However, I question the logistics of running through the street with falling teeth. Even if your MC doesn't want to seek shelter (and falling teeth would probably be painful), I wonder how well somebody could run when you have lots and lots of small objects littering the ground. I imagine it would be like trying to run on gumballs, except gumballs can break under your shoes, teeth should be more durable. I guess it depends on how many teeth are falling at a time. I'm assuming it's comparable to rain, which would likely stack them up pretty quickly.
Beyond that, it's an interesting set-up with a neat visual (that reminds me how underwhelming the scene in American Horror Story: Roanoke when a few small number of teeth fell there... and, come to think of it,
I don't remember that part of the story ever being explained in AHS -- it might have just been a cool throwaway moment). It doesn't really establish the MC, but it conveys a sense of setting and tone -- particularly in how unalarmed and dismissive the MC is of the event, suggesting (in a comical way) that this is nothing special within their world.