Discoloration on back of Martin Logan Quest speakers

Probably dust from the panels being energized. You can clean the panels and other areas without too much work. For areas other than the panels a dish rag with warm water will do it. 

If it's any area you are afraid of touching a soft painter's brush will work.

Make sure if it's near panels to de-energize the speakers long before dusting.

If its the painted MDF parts it could be mold growth clean as you would any molding item.

Thanks everyone, I'll follow these suggestions.