Sand and wax
If its made it through the anodized coating then,  the alum surface is starting to corrode/react with moisture and salt air.   This happens to be my winter project for my 87'TR.  As my entire mast and boom has this problem I will sand the oxidation/corrosion down progressively to a 600 grit or better finish and then 2 or more coats of wax. I'll have to keep it waxed if I want it to hold up 

You should be able to do the same thing assuming your mast and boom were anodized with a clear coat.....