Is this even a green ability? Possibly not, though Bind and Voidslime do give hints that it might be alright. If I include Needle Strike in the final set, this would make sense with that, as well.
I also considered letting it shut off mana abilities, but that's probably too strong at this converted mana cost.

It certainly reads like a white ability (Arrest, Stony Silence etc), or perhaps blue. There are versions of it in green, but Bind was later followed by Squelch, and Arachnus Web is viewed as something of a pie violation.

I agree. In retrospect, I think I'd like to stretch the pie as little as possible when making this set, to see how doable that is when removing an entire color.