A white powdery buildup which can be easily wiped off probably isn't aluminium oxide, I think... Alu oxide is greyish and sticks to the metal very well. Also, the anodizing process which is used to finish pretty much all aluminum is simply a chemical means of building up a thicker oxide layer. Alu will sponateously oxidise in air in a matter of minutes.
I'd suspect that limescale is a more likely explanation. If it rinses off easily with vinegar, it's limescale.
As far as I know, aluminum itself is more dangerous than its oxide, especially when dissolved in water or inhaled as a fine powder.
If you're worried about alu oxide, don't drink canned beer, don't use tinfoil, and don't use alu mess tins or cooking equipment of any kind. Personally, I'm not that worried...
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