Black Dust on Alternator Housing?

If the car not driven much the copper slip plates that the brushes ride on will get corroded and turn green, the surface will be very rough compared to polished when a vehicle is run every day. the brushes will take a beating till they get polished up. If the belt is loose or out of align far enough to wear the belt it would be very noisy, squeals a lot. I was assuming this was not the case. So I ask is the belt loose, squeals, so on?