I had a Dyna Superglide that had this issue. I tried a multitude of metal polishes - from course to fine. Polish will make the edges of the fin smooth, but the discoloration remains. I even tried a Dremel with a polishing bob, with no luck. Without a pic of your engine, I'm speculating it's the same thing (oxidation/corrosion), which tends to increases with age. Keeping the bike clean, indoors, and away from chemicals are the best inhibitors. You also see this happen to Harley aluminum forks that are clear-coated (like on non-CVO bikes). Once the clear-coat is perforated, as with a stone chip, the aluminum is exposed to the elements and you see the oxidation starting to form beneath the clear-coat.