Can someone tell me what this is?

i don't know.. but the interior of a pollen sac would look different than that. Pluck it out, if you want. Pretty tough to assess this picture with confidence. seems a bit big to be a young, nascent, green seed, but could be wrong. when the seed starts to portrude i've never seen it so brightly green. Maybe an insect took a bite off a calyx and didn't like it? Looks like it was damaged somehow.

A Immature Seed. Something pollinated it along the way. Was it a nanner or a pollen snack from the same plant or from somewhere else is the question. If its just one seed or a few seeds. likely a rouge nanner and nothing to worry about it. Don't grow them seeds out unless you legit have nothing else as its more likely to do it again. If its a lot of seeds then their might have been a pollen sack on that plant and it hermied out or was close to another plant producing pollen. Pollen from a nanner is mostly inert and not that viable. pollen from a full blown pollen sack is much more viable. With out knowing its source its a wild card and just not worth it. Best to toss the seeds or eat them.

Looks like an immature seed to me