The dandruff u describe is normal. Leave it alone. The more you futz with it the more irritated it becomes which may cause horse to want to rub its high end. Then you have a much worse aesthetic than a little hidden dandruff.
Even when I show-groomed & every other square inch of horseflesh was coiffed to the nines, we never dared stir up the hornets nest of dandruff on the tailbone, lest you find all your braiding rubbed out the second you turned your back.
However, do make sure to periodically check your horse's tailbone for ticks if they are in your area of the country.
Kerry in NY