My horse has dandruff?!?!

I got a new horse a couple of days ago and have noticed she has what looks like dandruff in her mane.
She is not used to being groomed and I have been grooming her twice daily, I was wondering if this may be causing it. Although I did notice she had a little bit before we got her, it has increased considerably in the last few days.
If you have any helpful tips or hints on how to avoid/reduce/eliminate it they would be greatly appreciated!

Oliver had a dandruff bunny in his tail! It's was so gross, and was suddenly there out of the blue! What worked for me is washing it with head and shoulders, really used a lot, scrubbed it in there, let it sit for bit then hosed it out. Only took one washing and looked way better! I'm sure it felt way better too!
So I suggest head and shoulders treatment, see how that works, then maybe tree tree oil and massage it into the roots to help moisturize it?

Good luck!
It was so nasty doing Ollie's tail!!

I use Head and Shoulders shampoo when I notice dandruff on my horse. I don't have to use it very often, but it does work.