Dr. Bronner's soap causing "dusty" look?

I went no-poo for a few months using only baking soda an ACV, but it was leaving my hair very oily. I switched to Dr. Bronner's soap, and have tried Tea Tree Oil and Hemp Citrus. When I comb my hair, the comb looks like it is full of dust! Is this dry scalp? I'm not using anything except the soap, but perhaps I am not rinsing it out thoroughly? Thanks in advance.