The appearance you're observing is not a defect or a stain, but rather a characteristic of how the image translates when printed onto polyester fabric. Unlike smoother materials such as cotton, polyester can cause the printed image to have slightly different color saturation, contrast, and pixel definition.
This can sometimes give the impression of light spots, discoloration, or a "paint stain" effect, even if the original design is perfectly clear. This is an inherent and common aspect of the printing process on the unique material of our socks.