O.K....No joke. I'm only guessing....and this is a long shot to say the least.
The white stuff could be calcium, lime and magnesium. This could only happen if the head gasket/o-ring was breached. As cooling water is heated minerals come out of solution and solidify.
Under normal conditions in the cooling system this can't happen. The water is never heated enough and is moving too fast to deposit minerals on a specific surface.
But, if cooling water was able to enter the cylinder, then it would be heated....even evaporated...leaving behind the minerals that were contained in the cooling water. Fresh or salt will do the same.
The sea loses no salt. Evaporation leaves the minerals behind.
I don't think that's aluminum on your plugs.
Must have been going on for a while.
Check compression for starters.