Could be several things. Is it flaky ? powdery? sticky? Or is it little white dots attached to the ends of the hair strands? If it is the later ...it is your hair bulb. The Hair follicle is a little bulb that is under the skin. It anchors the hair to the skin. The hair follicle is where your hair actually grows from. The white bulb is where the hair was attached to the follicle. The white is where the hair stops producing melanin right before it naturally falls out.
Because we have dreads...our hair is caught into the knots. When we shed hair...it stays in the knots. Most of the shed hair still has the bulb attacked. So especially in dark hair ....the bulb looks like little white dots on and in the dread.
My dreads are covered in these. I have tried everything to get rid of them. But have accepted the fact that they are just part of my dreads.