DIAGNOSIS That's mineral deposit buildup (hard water scale), not damage to the pot. It happens from boiling hard water, especially if you let water evaporate and sit in the pot.
FIX It cleans off easily with acid. Fill the pot with a mixture of equal parts white vinegar and water. Bring it to a boil (without the lid), then let it simmer for 10-15 minutes. Let it cool, then scrub with a non-abrasive sponge. The deposits should dissolve.
IF THAT DIDN'T WORK For stubborn spots, make a paste of baking soda and water or lemon juice. Apply it directly to the spots, let it sit for an hour, then scrub. This is safe for aluminum.
HEADS UP Avoid using harsh alkaline cleaners like oven cleaner or bleach-based products on the aluminum interior, as they can cause discoloration or pitting. Stick to vinegar, baking soda, or mild dish soap.