Hi, firstly welcome to this forum (maybe if you feel like it you could introduce yourself on the welcome thread), it would be useful to have some more information such as where you purchased the fish, how long ago, did you buy just the one or are there others and if so how are they?
It could be a bit of ammonia burn occured maybe during transport. If so with plenty of clean water it will mend, bit of a worry as the same might have happened to other delicate exposed structures as the gills - is his breathing fast or laboured?