Probably lacquer
Some aluminum products are lacquered to prevent oxidation. Over time, especially on something that sees a lot of grit, like a dirt bike rim, the lacquer can fade and/or flake away. I've often had this occur on street bikes with cast aluminum wheels.
Use aluminum polish to clean off the wheel and give it a good shine. I've used "Mother's Mag & Aluminum Polish" with excellent results.
http://www.mothers.com/
As long as you keep the wheels clean after riding, oxidation shouldn't be a problem.