I have done lots of questing in Un'Goro crater and have never seen these guys until helping a friend to quest yesterday. Simone and the wolf were nearby when I was fighting something else and I'm not sure if my pet attacked one of them in the melee or my multi-shot got one of them but all of a sudden, Simone was gone and there stood 2 cleaners. Yes, 2 of them. I'm not a priest so it had nothing to do with the priest quest in my case. I'm a lvl 70 hunter. The cleaners were 63 gold elites and I was dead within a couple of seconds. I'm not sure the damage they dealt to me, but I had full health since the mob I was fighting didn't affect me. Believe me, I was completely shocked to see them appear.

Obviously a reference to The Cleaner from Labyrinth (the film)
Hoggle: ITS THE CLEANERR!!!!!!!!
http://en.wikipedia.org/wiki/Labyrinth_%28film%29

I have a new life's dream. Enter the area while a priest does the quest, make the Cleaner spawn, enslave him, and then go and tell King Mosh my way of thinking with him.

I had this chap come in and kill me while I was doing the The Balance of Light and Shadow quest. I am an Alliance Priest and every time any Alliance entered the area, The Cleaner would appear and take me out and not the other Alliance. When Horde entered the area, The Cleaner would appear and try to take them out. Unfortunately in my case, it was an Undead Mage and after he fireballed me he teleported out of range of The Cleaner to safety.

I believe the Priest is attacked by The Cleaner if someone of the same faction enters the area or attempts to heal the Priest from outside of the area.

If a member of a different faction trys to intervene, then they themselves are attacked.

Has anyone actually organized a demonhunt on this guy to see if he is killable, and if so what he drops?

wat type of demon is he?