You guys realize our blocks are less than 20% magnesium right, same goes for the valve covers and intake manifold.

Its corrosion from salt, people in florida and up north have the problem a lot. My car came from detriot (mfg rep. car) and had the same problem, uncoated aluminum (not magnesium) turns white when corroded by salt. magnesium turns to its natural color, dark grey when it oxidizes.

with the low magnesium content, it has nothing to do with the magnesium.. its all about the aluminum.

You can get rid of it two ways. etch the block with hydrofluoric acid. or break out a scotchbrite pad and get to work