Use some wd40 and that will help keep the rust from expanding. You need to keep the surface covered every few months.
Use steel wool if you want to clean that surface.
Wow, that's being anal! Whatever you do, do NOT go to the dealer about that. (I work at a dealer, I see people come in with silly questions like that..)
But, it's good that you ask. If you don't ask, you'll never know..
OP, I felt the same way when I was installing my Corsa and took a peek under the car. The rear differential looks like it's a 20 year old car and will freak you out. It looks rusted and what not. Certain pieces aren't coated so it looks like that. Feel free to clean it up and coat/paint it yourself.
Overall I think these and many other MFGs leave too much metal
to rust,esp under carriage. My car had rust under it on day one,why they
feel nothing cast iron needs paint burns me up