Yea, that's a green frog (Rana clamitans). I got one in with the feeder from Petsmart when I worked there. I got mine for free though, lol. When I brought mine home he still had a very small tail left on him, and he wouldn't eat for at least a week and a half. Just keep trying to feed him. They don't eat much, sometimes not at all, when they go through metamorphasis. But when he finally starts eating, he'll eat a lot! So just keep trying until he finally desides to eat!
I read somewhere that when they absorb their tails, they use it for nutrients, and that's why they don't need to eat during that time. Not sure if its true, but it makes sense to me.