I don't know how to fix it but my yearling has the same problem, only on one spot of his neck and in his mane. It looks gross but my farrier said its harmless. He suggested bathing him but we don't have a wash stall or heated water at my barn :(
Yeah it's really cold here too, I've been wondering how to get it out for I don't know how long now. My mare has a black mane so it shows up really well, but that's the only place she's got it. It make her look very dirty!
First of all, what are you feeding her? Diet is extremely important in making sure that your horse is even able to produce the oils in her coat to prevent the dandruff. For example, my Percheron/ Thoroughbred cross who has issues with dry skin/ dandruff, is on 3 1/2 cups of Enrich 32 (ration balancer pellets), 2 flakes of fescue hay at night (don't know the lbs), and pasture of mostly bermuda and fescue during the day. I used to feed her flax seed- which boosted the oils in her coat, but my feed store hasn't carried it for a while now.
I would definitely avoid bathing, since it will only strip the oils from her skin. What you need to do (after balancing her diet) is brush her at least everyday. Try to focus on currying, since it will break up the dandruff, increase circulation, and increase the skin's oil production.