Soda Ash is a harmless white powder that can form on top of cold process soap. It's completely harmless, but just not very pretty. The cause is believed to have something to do with soaping too hot or too cold and also from improper insulation. I don't insulate so I suppose that would be considered very improper!
I believe the soap heats up for the gel phase (quickly if you're using milk - which I do!) and then the surface cools down too rapidly causing the ash if the top isn't covered to prevent air from
getting to it.
This is a short video sharing my trick for preventing soda ash on the surface of my soap.
It may me very helpful for those who like me, don't like to insulate.