could be soda ash. Its hard to prevent, but sometimes covering the soap batter with something helps. I've heard something like saran wrap touching the soap especially helps. Some say specific ingredients make it more likely, and it seems smaller molds cause it frequently. There's various youtube videos on removing it including a soaping 101 one. Alcohol, water, and steam are all used by various people to remove it. I tried to steam some off recently but probably didn't have hot enough water, so instead I just used the water directly and my finger to help get the entire layer off.