The white powder on aluminum is pretty much like rust on iron or steel. Just clean it off and repaint if the finish is too bad to live with. As long as you keep your machine dry it shouldn't get worse. A lot of the time the white aluminum oxide will look worse than what the damage really is once it's been cleaned off.
Steve nailed it on the crazing. Hot/cold cycles. The metal expands and contracts at a different rate than the finish, the elasticity of the finish may play a part as well. A hard brittle finish will crack more/sooner than one that is flexible.
Rodney