I broadly agree with wellington. Don't try to apply human standards of clean to your tortoise. Staining from foods sometimes occurs. If it doesn't come off in a soak, don't worry about it
I agree with Wellington as well. I was wondering why we are even trying to clean the green/ stain at all. It's not hurting anything and not impacting your tort's functioning...eating, etc, so I'd recommend let things all alone.
If you try to scrub it off it's likely going ot be stressful for him, and the next time he eats it's going to happen again. If there's not something caked on causing issues I wouldn't worry. Maybe you could share a photo with us?
I had an Eastern Box Turtle named Daisy that loved blueberries so much her beak was always purple. It usually took until the month before blueberry season before it was back to its normal color.