Why does my dog have grey and white circles in their eye mucus?

Thanks for using Petco Pet Education Center, formerly Petcoach! I'm sorry to hear Cookie is having eye issues. Eye discharge like you are having can be due to a variety of reasons, including KCS (dry eye), a scratch/ulcer on the cornea, an eye infection, foreign material in the eye, or another underlying issue. If the discharge is persistent and is starting to make the eye goopy, I would commend she be evaluated by her regular veterinarian to determine the underlying cause for her eye issue, as some can cause permanent damage that would benefit from being addressed quickly. I would have her examined, have any necessary testing done (tear production, stain for scratches, pressure check for glaucoma if needed) and start on any treatment she may need to help resolve it. Good luck!