Maybe someone painted it when you weren't looking. It really doesn't sound like any normal corrosion I have ever seen and I've seen some weird stuff.
Do you have any pictures of it? What do your neighbors say? It is possible there is some local biological source. What do your zincs and thru-hulls look like? Is this only on the prop?
Is the prop a NiBrAl unit ... it is possible that the aluminum is oxidizing for some reason.
*It could be biological as the harbor was under going a major renovation and the site of the harbor was heavely compromised during the 1964 earthquake.
There is so much oil and other things surrounding the harbor as it was first a military base shortley after the war.
During the renovation when they got below 2 ft of ground it was like an oil well. They had to boom the entire harbor.
The buzz was that there is so much contamination that nothing could be done about it so they just ignored the issue.
I was told something like 50,000 gallons of*aviation fuel was leaked from a ruptured tank farm.