That definitely isn't a threadfin (chaetodon auriga), it's too plain- it looks more like a klein's butterfly to me, but not exactly. Did you see it eat at the store? I worked at ***** back in the day, and when we ordered those assorted b/f's, they didn't usually adapt to aquariums if they didn't arrive eating flakes or frozen. Hopefully you can convince it to eat flakes, if not I might try some sponge formula or one of those frozen foods for angels and butterflies. Good luck with it, though, it's pretty.