This is what I usually call a fish tumor. I've had customers bring them in with them. and I've once bought a fish with it before, It was part of a group of 6 cupidos. In all the cases bringing it to a tank with less than 40 nitrates and high quality food, shrunk it down to unnoticeable size in a month of two. You can still see a small white spot on the fin usually, but definitely shrunk. I haven't been able to observe this for any longer than 6 months. So I am unsure if it'll ever completely disapear.