It was anodized when new and that wears off with time. The good news is that the dull gray appearance you see keeps it from degrading further, it's a layer of oxidation. You can remove and replace it, or have it re-anodized, neither of which is cheap by any measure, and the problem will be back. Perhaps the best solution is to paint it. You must remove the oxidation, etch it, prime it with an aluminum specific primer, and then paint it. The results can be outstanding but time comsuming.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.