I'm assuming it's a copper mouth bit. I would try soaking it in vinegar-water, mixed pretty strong, and see if the green comes off. If it doesn't come clean, I would just get a new one. I don't think the "noble rust" would harm the horse, but I've never seen it on a bit, and better safe than sorry. I'd be super leery of using a commercial/chemical copper cleaner, those are pretty nasty to have any possibility of being near a horse's mouth, IMO.
I've seen bits with dried grass slime on them so thick that it could pass for patina, and I deal with that with a soak in hot, not boiling, water and (I use Palmolive) dishsoap until the water cools, then chisel at it with an old toothbrush and toothpicks. Nast work, but I have clean bits afterward. I use enough dishsoap that when it's mixed in I get a layer of foam on top of the water, like 2 good sqeezes of the bottle.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown