Hard to tell from your picture....could be condensation? My Harley used to get that when you had cool mornings after humid days.
I have similar spots on my fuel and coolant temp gages as well as on the face of the tach. They seem to move around while I drive. The dealer had never seen anything like it. Wants about $800 to remove and replace the gage set. has anyone had experience removing and cleaning the gages?
