Have I rendered my aluminum baking sheet pan unusable?

On Sunday I made bread pudding. It was in a 13×9 non-stick cake pan. I placed this pan on an aluminum pan and added water for the necessary water bath. When it was done and I removed the 13×9 pan, I saw that the aluminum pan was all discolored. It’s kind of blotchy, with shades of grey from light to dark. I have not tried to clean it with anything because I don’t want to make things worse.

Any idea if this is permanent? Can I still bake on it if I use a silicone sheet or parchment paper? Any ideas on how to get rid of the discoloration?

Many thanks for any help you can provide!