Far as I can tell Mist is more of a term used by Viera to describe a thick palpable amount of Aether that is present in their home forest. Rather than something different, could be more akin to like a mana potion bath and essentially aether doping after too much exposure from beings sensitive to it. i.e. Mages being more affected or affected quicker than say Fighters. Multiple times in the story so far has concentrated/prolonged exposure to energy/aether sources has been shown to have adverse effects, whether it be Y'shtola and Thancred in the Lifestream, or production of artificial echoes with Fordola and Zenos, killing Ascians with concentrated unaspected aether and the ability to convert life force into aether causing death like with Louisoix and Moenbryda.