This does sound like mites to me. Mites can be very hard to see with the naked eye. If you can get a handheld microscope, you can see the mites easier. You can take a q-tip and wipe it on her head, and look at the q-tip with the microscope. I did that with one of my fence lizards, who just didn't look right, he looked pefty and rough. I found a few mites I had not been able to find by the naked eye, and treated everybody in my collection with Reptile Relief, and thoroughly cleaned their habitats, and sprayed Pyrethrum all around (Really waged war here!)
Its been several weeks now and no more mites!
When she soaks in her bowl the mites go to the top of her head and hide between the head plates. The grey dust is their waste.
You can put the Reptile Relief right on her head with a q-tip, being careful to not get it in her eyes, although I have had wiggly creatures manage to get it in their eyes with no ill effects. The Reptile Relief works by dissolving the exoskeletons of the pests, but is harmless to reptiles. Not for amphibians,though.