What is this? I discovered it in a heating vent within my home on drywall, not metal. I also noticed some areas around nail heads that have a similar coloration, but there isn't as much of it. I've only ever seen this kind of thing outside on rocks and wood. It seems to be a permanent fixture on the drywall and isn't powdery at all. I haven't attempted removal as I have no idea what it is.
Please see the photobucket pic for an image: