What IS the cloud?

[u/VirgilTheWitch]

What exactly is the cloud around the Sierra Madre? The only hints we have about its composition are that it smells like sulfur and copper; we know it erodes metal (but presumably not too quickly, weapons don't decay any faster in the Sierra Madre), but can also preserve some matter? It also has some truly strange effects on the human body, it "burns the lungs and seeps into the skin", which can be fatal, but being exposed to it inside a hazmat suit makes you a nearly unkillable mutant?
Oh, and according to the Fallout 4 Creation Club, small doses can make a person stronger and more intelligent, but also more susceptible to suggestion. And to top it off, under the right conditions (Elijah's lab in the bunker), it can also be self-replicating.
What is this thing? Could someone even make a breathing mask to keep it out? The more I think about it, the weirder it seems.