Powdery substance, brown spots on Maxillaria

[Vrana]

So I'm finding my new found orchid hobby as frustrating as it is rewarding. Gah. Anyhow my latest issue is...

I recently noticed the Maxillaria Tenufolia I got a couple weeks ago is covered in a fine whitish powdery looking substance, which is concentrated toward the center of the leaves. It almost looks like dust, and when I wipe it off it turns a pale brownish-green color. Is it mold? A bug? What do I do about it? Unfortunately I didn't get a picture before cleaning it off (I wiped the whole plant down with q-tips soaked in a mix of water and hydrogen peroxide). My nearby plants don't seem to be affected, though I isolated it just to be safe.

I also noticed as I was checking on the roots that there's a p-bulb in the center, beneath the 'soil' line (for lack of a better term - it's planted in orchid bark mix with some sphagnum moss) below where the roots begin, that seems to be rotting. It's turning a brownish-black, as is the leaf coming out of it. There's also some dry brown spots on a couple of the p-bulbs on one side of the plant. These were there when I got it, but should I be worried? Does the plant look okay otherwise? I'm very much a novice here...

I've included some pictures. Thank you in advance.