I agree with FB that harsh abrasives should be avoided. A fine Scotchbrite pad should remove the loose stuff, followed by a good paste wax as a protective layer. Don't try to get a mirror shine, just get the joints clean and free of the buildup.
Aluminum does spontaneously form an oxide layer that self-protects it but the layer is thin and can be damaged by salt or other chemical contact and discolored by prolonged water exposure. A much thicker and more protective oxide coating is created by "anodizing" but that's not a DIY process, particularly not on a built-up frame.