It is very common to find a thin layer of white powder on the top of your batch of soap. This is called “ash,” and it is harmless. It is essentially the minerals from the water that have collected on the surface. If you have a thick, sparkly crust on the surface of your soap, you probably have a caustic, lye-heavy batch. But a thin film of white powder isn't a problem.
Using purified water is the main way to limit the formation of ash. But even soap made with distilled water will sometimes have a layer of ash. It can be because of the composition of the lye, the way the ingredients work together — sometimes there is no identifiable reason.
If your soap has a crust of lye crystals, you must handle it wearing goggles and gloves as it is caustic. You may have added too much lye, stirred improperly, or the lye crystals may be the result of any number of other factors. The soap is probably lye-heavy and will never cure properly.
Placing plastic wrap on the surface of your poured soap is one way to eliminate the ash layer. Try to let the wrap cling to the surface on its own, rather than pressing, as you can add lumps and bumps to the surface if you push down too far. Peel it away when you are ready to unmold.
You can also remove the ash layer by hand. When you cut your bars, you can use a cheese planer to cut away the ash. Some soapers set up their cutters to take off the layer during the cutting process. Or you can just leave it. It isn't harmful, and it is an indication that the soap was made by hand. It is ultimately an aesthetic choice.