The Glow Plug Relay circuit board should not have any kind of corrosion on it. It sounds like someone has done some work on it. There should be a thin layer of conformal coating, available at electronics stores, to prevent this.
If someone has done some work on the board, they have to remove this coating to solder. They may have taken a shortcut and not restored the coating.
2108-12S Techspray | Mouser