This is harmless but not pleasing to the eye. Salty foods allowed to stand in pans may pit the metal.
Pitted pans are safe to cook in, but hard to clean.
Water with iron, calcium, or magnesium discolors aluminum pans. So does boiling eggs or unpeeled potatoes from the iron and sulfur they contain. Do not soak aluminum utensils for long periods, as alkali in detergents will discolor the finish, sometimes permanently. Pans may become warped if water is added when they are dry and overheated, or if very hot pans are placed in cold water right after removing them from the burner, as in cooling food. Wait a few minutes to do this.