As Anemone mentioned, probably a very young Prionurus species of tang. I'd guess P. punctatus just because I know juvies start off yellow and can sometimes have a hint of a vertical bar through the tail. P. laticlavius might do the same, but I've never seen a juvenile of that species, so can't say for sure.
They're interesting fish- different from the other tangs. I think they're highly underrated in the hobby. Keep in mind these are not collected from tropical water, but from more temperate locations. It may do alright at the usual reef temps, but will be healthiest in slightly cooler waters (think upper 60s low 70s). They're super active fish that can grow 1.5-2 feet long, so make sure you've got plenty of open tank space. They'll eat just about anything, but large amounts of greenery is a must.