I had a large bag of potting soil. When I opened it there was a thin layer of white mold on top. I used it to re-pot a Peace Lily. Now I am thinking maybe I should not have used it. What is the answer?
It's probably not the end of the world. Put plenty of light on your plant (and its soil) and the light should inhibit the mold if it's pathogenic. When the plant is established it shouldn't be a big risk, unless it hardly gets any light, but I've never grown Peace Lillies. I don't think mold in and of itself is usually a big problem, but moldy soil may also have pithium and stuff in it. Maybe not, though.
Mould, or mycorrhizae? Its more likely to have been the latter - mould would look fluffy, whereas mycorrhizae is more root like, on examination. It's likely the compost you bought was made from recycled materials, and mycorrhizal growth is normal - its there to break down any elements within which are still not quite composted, particularly small woody parts. It shouldn't cause trouble in your pot, though I wouldn't deliberately include any I could see - some strains of mycorrhizae are not beneficial to plants, but the vast majority are.