Sounds like chick poop to me. I'm not trying to be facetious. Mine always have green blobs on their butts and in their litter when they're new...it's their poop and it's usually green for the first 24 hours until they start getting regulated and eat chick starter, drink water, etc. and then it turns beigish brown and white.
It could also be what I call the "poop sack" which is also normal in several shades of yellow, beige and even greenish. It sometimes comes out along with the chick and they'll drag it around the incubator until it eventually falls off. It's just the excrement from the chick as it was growing in the shell.
If it's small and greenish and on her actual vent or near it it's probably poop, if it's yellowy tinted green and looks similar to what you blow out of your nose when you're sick - and it's a touch lower than the vent it's probably the excrement sack as that is where it attaches. If it is the excrement sack, whatever you do, don't pull on it, as you can seriously injure and even kill the chick by doing that. It will fall off on it's own when the opening begins to dry, she was perhaps too moist upon hatching and it just wasn't dry enough. Either way it will take care of itself and then you can toss it in the trash.
Or it could be NEITHER of those things and something strange - a picture would help.