My chicken has dandruff or something similar

It looks like she has scaly leg mites, you can tell because the scales on the leg are upturned we've been going through a case of it as well. We've been spraying WD-40 on the legs then applying it with bagbalm

 

I can't tell if your chicken has lice eggs or what on her head, but she definitely has bad scaly leg mites. For scaly leg mites, you want to gently scrub the legs and feet scales once a week with an old toothbrush and warm soapy water. Then apply castor oil, vegetable oil, or vaseline once a week and rub it under the scales for several weeks until you see new scales growing in. For lice or mites on the skin, use permethrin or Sevin dust every 7 days until the eggs are gone.Don't bother with DE because it won't help with an active infestation. The coop has to be cleaned out of bedding (nests , too) and sprayed with permethrin, then clean pine shavings added. Here are some good links to read about how to recognize lice eggs and mites:
http://anrcatalog.ucdavis.edu/pdf/8162.pdf
http://www.the-chicken-chick.com/2012/08/poultry-lice-and-mites-identification.html
http://edis.ifas.ufl.edu/ig140