Elder Willow is one of the Dryad Elders, from what we saw in #276 the different Elders represent different natural plants or such, e.g. Elder Willow has what looks like a willow branch.
She is a calm and centered Elder much different to Elder Fungus or June, from what is known she is the only one to make June apologize for something she did, because of this Maxx was shocked and ask her if she was a wizardess for doing this feat.
June: "Of course they will. They're elders, it's their job to disapprove of things."
Willow: "June! That's no way to talk about your elders!"
June: "It's true, though.
Willow: "...Be that as it may, you should still apologize.
June: *Very quietly* "...Sorry"
Willow: "What was that?"
June: *Slightly angry* "I said I was sorry!"
Willow: "Thank you"
Maxx: *Still shocked* "Are you a wizardess"