The Service Manual calls it the Bumper Skirt. If you do not have the manual, and cannot get it from this website, then somehow get a copy of pages 20-49 and 20-50, to see what is involved. You have to fold back a floor trim panel, remove fender liners, under covers, 10 nuts, and the bumper assembly. Apparently, you cannot just remove the Skirt itself until the whole assembly is out, but I have never personally done it. The book says to get help lifting the assembly, and don't forget to disconnect the license plate light connector.
Happy painting,
Bill