I know this thread is over a year old, but I found myself referring to this thread just this week (6.24.20) for help so I thought I'd post my experience in case anyone else is looking at this these days! I'm growing rabbit corneal epithelial cells and the original passage looked great the whole way through, I passaged about a week after plating, and 3 days into passage 1 contaminants EXACTLY like the ones in OP's photos began popping up. At first it was just a couple in one plate, but after a couple more days all 3 of my plates were covered. I knew it wasn't a live contaminant because the cells remained healthy and it didn't move, grow, or change; but still more and more showed up. After 5 days of this, I finally noticed that the contamination was worst around the ledge/rim on the bottom of the plate; which didn't make sense because why would a rim on the outside of the plate affect debris on the inside... and that's when I finally wiped down the entire outside/bottom of the plates with ethanol and sure enough all contamination was instantly gone.
So, if you've come here because you're seeing weird things in your plate, do yourself a favor and start by wiping down the outside/bottoms of your plate first. Might save you a lot of grief!