I've seen similar things in cell culture before and it hasn't been a problem. Could be that something stuck to the plate from incubator shelves or anywhere else it was stored. Were you able to tell if it was inside the plate or outside? Determining if it is among the cells or on the lid can also be done by comparing microscope focus. Notable contaminations like bacteria and fungus have some other tells that should emerge soon if that's what it is, such as odor and fuzzy appearance respectively.

Not contamination that could ruin your cells. Most likely industrial dusts that got stuck during the making of the plate or when you opened the packaging. It’ll disappear once you wash and spin your cells a few passages later.