Occasionally, dusty-looking blemishes may appear on your wellies. This is called 'blooming' and is completely natural, this is the wax included in the rubber rising to the surface when exposed to changing environmental factors.
It is a natural wax, as is the rubber used, and so is not harmful. It is added to increase the longevity of your boots and keep it flexible whilst preventing them from cracking.
To remove the wax from the surface and restore the original colour of your boots, wipe clean with a warm, damp cloth. For a longer lasting treatment, they can be polished with a rubber cleaner and dressing, which the retailer should be able to apply for you free of charge.