Soda ash is a harmless white powdery coating that sometimes forms on the surface of cold process soap. It is caused by sodium hydroxide in the soap reacting with carbon dioxide in the air before saponification is complete. It does not affect the soap's quality or safety — it is purely cosmetic.
The best prevention methods: spray the surface of freshly poured soap with 99% isopropyl alcohol immediately after pouring, then again 10–15 minutes later. The alcohol creates a barrier that prevents carbon dioxide from reaching the lye.
Other prevention techniques: cover the mold with plastic wrap (pressed directly onto the soap surface) after pouring. Use a water discount (less water = less free lye available to react with air). Pour at a slightly thicker trace so less lye is exposed to air.
If soda ash forms on your cured bars, you can remove it by: steaming the bars briefly with a garment steamer (wipe immediately with a soft cloth), rinsing under cold water and rubbing the surface, or trimming a thin layer off the top of the bar with a vegetable peeler.
Some soap makers embrace light soda ash on rustic-style soaps for a textured, vintage look. If your design is meant to showcase colors or swirls, prevention is worth the extra effort.