It's another case of tumor to me. Whether benign or cancerous, it's impossible to tell unless you want to charge a fish vet well to have this looked at but it's not really contagious so there is no reason for you to terminate the fish at all. Aesthetically, it's bothersome to look at but I suppose if it is eating just fine and the tumor is not near the gills which would be a very critical area, then the fish can live for as long as its health can permit it. If the tumor were external, it could be surgically removed but internal ones are far too tricky and this is without ending up killing the fish in the process instead.
