Wheel fender. I had mine off being hit by a busted tire on highway. Very easy to buy a new one and put it on by yourself

Found it cheaper here, had to replace the same thing on my cx5. Hope this helps

https://www.carparts.com/search?q=mazda_cx-9_fender_flares_rear

I got one from eBay in like new condition for about $80 with shipping

How in the world is something like this not covered under warranty