my 12" gso had the same thing zander except a good clean fixed it right up
as for the 'ohno dont clean it' brigade, in my opinion leaving dust etc on the primary especially when its going to be getting dewed up is asking for problems.
all i did was give it a quick blast under the tap (in the kitchen sink) to get rid of the loose stuff, let it soak in warm soapy water then rinse with the tap + demineralised water, no touching the surface = no damage and from the times that ive cleaned my mirrors using the method above it has come up spotless, the thought of dragging anything across the face of the mirror worries me a bit