Calcium is actually pretty chemically active; I wouldn't be too surprised if it makes an environment that's inhospitable to molds. I had recurring root rot problems a while ago, but I just stepped my watering down a bit and it sounds like you've got that under control. Epsom salts are a magnesium compound, and my understanding is that that's why they're effective for a lot of gardening problems -- not so much that plants need magnesium, but that fungi can't tolerate it.