everyone sheds skin cells..we have skin cells on our scalps. loose hair gets brushed on a regular basis. Brushing massages the scalp and takes most of the flakes away. You have spent most of your life doing this. Your scalp is used to that daily attention.
Now you have dreads. Can't brush any more but the skin cells are still shedding. So the dead skin just hangs around a little bit more. And the scalp gets a little upset because it isn't being stimulated.
Temperature changes also dries the scalp. So going outside in the cold and then walking into a heated home will have an effect on flaking.
While you are washing your hair...that white gunky stuff is skin cells and your natural oils that the water and cleaner is trying to break up.
If you are finding white stuff under your nails when scraping a dry scalp, it is most likely just dead skin cells.
I GENTLY scratch my whole head with my nails before i wash my hair to loosen up that skin. I alternate between shampoo and BS ACV. Every other week i mix about 1/4 cup baking soda with warm tap water in a gallon milk jug. I shake it real good to make sure it is all mixed up. Then i slowly pour it over my head...concentrating on my scalp. I let it sit for about 5 or 10 minutes. While it is sitting i again gently scratch my scalp. I rinse it well under the faucet...again concentrating on my scalp. In the meantime i have mixed up about a cup of vinegar , 5 to 10 drops of tea tree oil with tap water in the milk jug. I pour this mixture over my head. i gently rub my scalp for a minute then i rinse my head again under the faucet.
The next week i wash my hair with shampoo, concentrating on the scalp still!
I still get some flakes sometimes, but not too bad.
If you are using a sea salt spray, keep it off your scalp as much as possible. it will dry your scalp and make it flakier.
The longer you have your dreads the more your scalp will get used to the new routine of no brushing
I hope that helped