I concur about the Oxidized aluminum surface, my car sits covered 95% of the time and I get that stuff too on aluminum surfaces, wipe it off or a light wipe down with
WD-40, nothing really to worry about, CD-2 engine detailer will work too if you can find some at any local autozone, but be warned its been cancelled so all you will find will be the remnants if any... is your garage cold & damp?