What kind of substrate? I've seen little white diddlies before on reptiBark but pretty sure they weren't mites - certainly mite problems didnt develop from it. Mite larvae are damn hard to see with the naked eye but they are pale and they can walk - but if you had those I imagine you'd have seen the mites capable of laynig them. Check well around the eyes and under the chin as they seem to really favor those moister spots.