It's soda ash. Yes it can be thick coating as opposed to a light dusting. And no, it has nothing to do with the use of TD because I've had orange soda ash, purple soda ash and brown soda ash. I haven't figured out a rhyme or reason myself...too hot/too cold, in the house/in the garage, alcohol/no alcohol, too thick/too thin, covered/not covered, EO/FO, during saponification/during curing.
My orange soap was an EO/Mica. 10" loaf along with 30 'samples' in small, round cavity molds. Room temp, poured at a medium trace, left it uncovered in the kitchen. The ash on the sample soaps was hugely thick...after rubbing off what I could I tried to plane them, but ended up tossing them in the garbage. I easily lost a third of the loaf.
The purple soap was FO/Mica. 10" loaf, poured at a very thin trace, put in the garage. I had the lightest dusting of ash on top when I went to cut it, but it was the thick ash that developed on one SIDE of the bar during curing that left me puzzled; it's the only soap that that has happened to.
The brown soap was FO/TD. 10" loaf, poured at a thin-medium trace. The FO discolored to brown, so poured off a small amount for a swirl, added just a bit of TD for contrast.
It should be noted that the thick ash reminded me of marshmallow.