i dunno if this is just another "vanilla thingie", though according to wowwiki.com Ghost Mushrooms should exclusively spawn inside caves. In Hinterlands they actually spawn outside too.
As far as Azeroth goes, Ghost Mushrooms can be found only in The Hinterlands (in the cave beneath Skulk Rock, and in the cave near the altar in Jintha'Alor), Razorfen Downs, Maraudon and Dire Maul. They grow exclusively underground.
And after all the stories of perma bans i wonder if gathering them results in a perma ban?
Screenshot
Another thing: I might be wrong, but i found not a single one in Jintha'alor yet, dunno if they are missing, that could be just bad timing tho.
Greetings Ulti