They renamed this in an older update, but haven't made it consistent in all contexts. (Like bison grass/aromatic grass at slightly higher levels). For example, to do your level 20 craftsman quest, the NPC in Varanas asked for Mountain Poplar in the quest text, but she actually accepted Mountain Demon Grass and let me finish the quest. You can still craft the recipes using either name, so it shouldn't hold up your crafting AFAIK.
PS: Does anyone here besides me think Mountain Demon Grass is a bizarre name for this herb? 
I know Poplar is a tree and it doesn't look like a tree in game, but demon grass? Should we start a petition to rename it Mountain Aloe instead?
Edit: Nevermind, I just saw in another thread that Demon Grass is the old name for Mountain Poplar. Now I'm just confused about why it got back into use in game recently.
