I posted a message too about this a few months back, did not get many responses. But it was just like yours and just out of the blue, the only thing I can tell you is it just finally went away on its own. I just wiped her face down everynight with a clean damp papertowel. The Vet gave me wipes, but I didn't care for them, they stunk and were dangerous for the eyes (all kinds of warnings about getting into the eyes). I am guessing it was allergy related.
My Darla gets that to as i just wash her face with a damp washcloth and it fades away as i'm always cleaning their faces especially around their eyes i just did their nails again but,i just wash her face and like i said it fades away.