Quite likely he was a hitchhiker. There are some recent records from SW Indiana- Posey and Vanderburgh counties- but that's still a long ways from you.

Gray treefrogs are often green, especially when young, but they will still show the characteristic white spot beneath the eye, even as metamorphs:

Keep an eye out for more greens; these guys have been establishing themselves in new localities throughout the area just north of their established range in recent years, and you may have a heretofore undiscovered population near you! In any case, this guy may well be worth a distribution note in Herpetological Review. You'll need the ID verified by a herpetologist, and you will likely also need to deposit a copy of your photo in a recognized museum collection. I don't know that Indiana has an active herpetological collection; you can try contacting a curator at the University of Michigan Museum.