Have you tried IPA with a magic eraser? The ME adds mechanical action (think light sanding) to the solvent power of IPA. If that doesn’t work for you, increase the mechanical action by going to 0000 Steel wool in place of the ME.
The texture makes it a bit tricky IMO...
I’d use a Dawn solution (careful not to get on painted surfaces or you’ll remove any wax), a soft bristle brush and several washes. For really stubborn areas try and magic eraser (lightly) but again avoid the painted surfaces.
Then 303 To keep it...
That looks like a failed component to me. You can see where it was once attached.
IMO - in addition to getting your dealer to fix it, you should also contact the NHTSA. It seems the whistle blowing welder that some on this site have questioned was indeed correct.
You’re very lucky to not be hurt.