Too much sebum/ scalp grease.
Help! Save My Dreads
I'm not sure mine was exactly this bad(or maybe the dreads just make it look worse)but before I had dreads I had this problem. Like yours there wasn't any flakes but this white paste covering my scalp and stuck to my hair. Especially if I didn't wash my hair at least every three days.
This isn't something I would recommend doing with dreads but I didn't put anything on my head but water for nearly a year and brushed it every day to distribute the oils. It basically allowed my scalp to reset every thing and I haven't had an issue since. Admittedly things were pretty ugly for the first month but it evened out eventually.
Hopefuly you can find a way to fix it without loosing your dreads.