Phalaenopsis with black roots, yellowing stem-help!

[JessieC]

I bought a gorgeous phal at a local farmer's market about two weeks ago-over 30" high, six huge leaves, and four flowers at the time. It now has six flowers, though the rest of the buds dropped off. I may have watered too soon after I bought it, though I've gotten better about that. However, many of the roots near the stem look black and have now started growing this white fuzzy mold. One of them feels hollow when I poke it, though the others feel firm. Today I noticed the actual stem, near the base, has a quarter-sized yellow patch, though the leaves look fine. I treated with peroxide but I am wondering if there is anything else I need to do.

I have never had an orchid before, so I'm not sure what it's planted in, but it looks like moss. And there are a ton of very green roots fit very snugly in a plastic pot, which I set in a bigger pot with some fake rocks because I was afraid it would tip over. I don't want to kill this gorgeous plant, so any help for this beginner would be great!