Honest question because I haven't kept up with Warcraft lore so maybe I missed something, but does Malfurion use arcane magic? Last I heard arcane was pretty explicitly a no-no in Night Elf society and is the entire reason the Blood Elves exist. They were Night Elves that dabbled in arcane magic, got a bit addicted, and were sent away to the Eastern Kingdoms to sit in a corner and reflect on their decisions.

I know for a fact at the very least his QWE skills and Tranquility are all nature magic. I don't know for Twilight Dream, but I would guess the name of the skill references the Emerald Dream, which is a very druidic thing and not arcane at all, which would mean that it's Nature magic. I only played Feral Druid (and not very much of it) so I'm very unfamiliar with druid spells.

That's just his HotS kit, though. It's possible Malfurion uses Arcane magic in lore and I missed it, but I'd be surprised.