A definitive answer to your query requires a real chemist. But do remember that various compounds are dissolved in the water you are boiling. You are probably observing the reaction of that material with the aluminum. I have encountered this on occasion; all I can say is that it cleans up readily. I have used aluminum pots for decades, and all of them are as shiny as when they were new.

Stainless cookware is heavy, strong and cheap; aluminum is light, cheap, and not incredibly strong; titanium is light, strong, and expensive.

Light, strong, cheap - pick any two.