Are you referring to the pink blotch right at the end of the tail?
I'm not sure what that would be, personally I would keep an eye on it for a few more days and see it it improves or changes, before taking any action. Make sure the water is clean, and keep up with water changes while it's being monitored.
Hopefully it will improve without any intervention.
A little update I went to the store she got him for some fish food today and they told me this guy got beat up by a gold nugget who used to be a tankmate of his