I am in Hondo's camp. That looks like it could be a lost lamination peel. The 35%
War Nickel alloy was new during WWII, and the Mint had problems mixing it thoroughly. That could be a bright patch of copper, silver, or manganese underneath metal that flaked off. I see a grainy, parallel pattern that is a good indicator of laminations. Are there any other areas nearby that are bubbling up or flaking?
It could also be a bit of surface that was scraped, stained, or something else entirely.