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?
It was so nasty doing Ollie's tail!!