How to get rid of white rust on the bottom of aluminum shower door?

magic eraser...cost $3.99 at your local grocery store. I swear by them. You might have to use a little elbow behind it but they work.

Sounds like calcium to me. A little CLR will take care of it

I would use white vinegar; It takes away lime deposits. Rub some on and let it sit for 5 minutes. You might gently use a steel wool pad very gently if it is in an area that won't show.There is also something you can buy at a hardware store that removes shower stain and it works like a miracle. It's only about 3 bucks for a quart.

it is oxidation but it is not rust scrape it off then buff it with a buffing wheel than give it a clear coat with some spray lacquer