Green poop can definitely be caused by differences in treats. If your hedgie normally only gets 2 mealworms and then one night has 8, it can cause green poop because that's not something they're used to getting. Same goes for new treats. I would just keep an eye on him for a few days and not introduce any new treats until it clears up.
i was sooo nervous just giving him a treat period. went with the banana and he loooooved it. gobbled it down.... put then out comes green poopie i've tried mealworms twice with him and he would give them a good sniff then take off running in the opposite direction, lol. and yet he told off a great dane!!! i've heard ppl talking about mealies.... are those just a nickname for mealworms? i've also heard mention of them liking crickets... i'll wait a few days to try any more treats with him though.
You can most definitely try banana again. Give him a few days to see if it clears up, and if ti does, it means he just was not used to that treat.
Try giving him another VERY SMALL piece of banana in a couple days after his stool is back to normal. An by small i mean just a sliver, almost none. If his poop goes green again after even the tiniest amount, then he probably should not be fed it any more.
If his poop does says a good color and texture, over time you should be able to feed a little more banana. Remember it is just a treat, and he should not be eating a human sized bite. A piece about the size and thickness of a dime is enough for a treat at the end. I would start with 1/4 of a dime a day, and slowly increase to 1 dime a day, and then eventually you could get 2 or 3 dime size pieces at once. It is all about moderation.