I guess she's not having enough sunlight, and she's lack of calcium.First, you need to use a brush and see if the white thing can be brush off, if not, then do expose her for more and longer sunlight. If you have a UVA light, this can help. I have this light and they like staying close to it when they are taking a rest and they will be healthier. Calcium supplement, if you can find, is also help.
If the condition continues, go to see a vet. Hope this helps!