Side coming apart
 

See my attached picture. I know due to rust is the reason. But any way to keep it from not falling off?
https://cimg6.ibsrv.net/gimg/www.hyu...c6202beea3.jpg
Sorry the car is dirty... definitely needs a car wash.

Very common especially if you drive where a lot of road salt is in use.
The dust gets packed behind the outer surface and when moisture gets to it , it goes to work rusting the unprotected metal. The fix is to have the area cleaned and have a patch installed and finished to secure the upper plastic section of the rear bumper.
The cover that is missing to protect the wheel lugs from corrosion will get even with you if you should ever need to remove them on the road with a flat tire.
It is only a piece of plastic, but does more than look nice!