Pics, please. If it's white fluffy mold it's usually harmless on the soil, and a spray with 3% hydrogen peroxide will kill it. You won't want to spray your buds with that, especially indoors. However it is usually a sign that you're watering too much. You could also put a layer of horticultural sand on top, it'll keep the surface from being too moist, and if you move it aside before you water very little of it will settle into your soil. Are you indoors? If so I would suggest posting this on the indoor forum as well to get their methods of dealing with mold. The methods will vary greatly from outdoor methods.