What causes a milky, dull, chalky coating to appear on aluminium? How to clean it off?

The cause is most likely due to the process of oxidation. Do not worry as there are steps that you can follow to get rid of the chalky coating.
  1. Use a soft nylon-bristled brush to clear the surface of the aluminium.

  2. Combine mild dish detergent with clean water.

  3. Dip a sponge into the solution and scrub the part of the oxidized aluminium, applying moderate pressure with even strokes. Make sure to wring and dry the sponge, then rub it on the aluminium again to remove any detergent residue.

  4. If the oxidation is still there, apply aluminium etching cleaner with stainless steel wool. Be sure to wear gloves when doing this step. Rinse away the cleaner with a sponge and clean water and allow it to air-dry completely.

  5. Lastly, give the aluminium a final clean-up with a microfiber cloth dampened with denatured alcohol to ensure there is no leftover residue.