I am thinking perhaps the planting mix you added to the garden soil had lots of good organic matter and now the white mold is just the by product of all that organic matter breaking down the material in the soil. Working the planting mix into the soil to a depth of at least six inches will help these filaments to disappear and should improve drainage. The slow growth of your bean seedlings may be due to the weather. Beans are a warm season crop and cool night temps can slow their growth. Is your soil moist when you dig? Water should run through the soil and not keep it soggy and damp. I suggest you take a bucket of the soil to a Master Gardener clinic so they can better analyze the situation. Good luck and Keep growing, Marianne Binetti