What is the curved plastic piece that is just above the tires?
I need to replace the one over the rear left tire, but I don't know what it's called.
Some kind of molding piece?

 

if you are referring to this area it is a wheel well or splash shield or wheel well liner .
You did not post your exact year Mazda so can't help much more than this. It's always a good idea when asking a service-related question to give at least the basic information about your Mazda or any vehcle just like as if you went to a service shop.

 

2020 CX5. It's the piece that attaches to the body, above the tire. Maybe 2 inches wide?
It looks like a C.

 

IDK you need to go look on Mazda real OEM parts under you vehcle as I suggested?
Try this: ??
2020 Mazda CX-5 Panel(l), wheel apron. Fender, panels - KB7W-54-210 - Genuine Mazda Part\



Or call your Mazda parts department give them your VIN. They can look it up and send a picture to your cell phone.

 

Is it this?
https://www.partsgeek.com/catalog/20...e_molding.html

 

That's on the outside of the wheel well.

Maybe you should post a picture indicating on your Mazda what you are needing to replace that is damaged or missing?

 

Yep....