Can you get a pic of this? It could be a parasite if it's on the gills. What are your water parameters, ammonia, nitrite and nitrate?
Im not positive but some of the things it could be is Nitrate poisoning, (systemic infection) Ammonia or Nitrite accumulation, Parasitism, Bacterial sepsis
So, for red or injected fins, you'd test your water quality, and if that's fine, you'd probably still be well advised to do a major water change and go ahead and salt the system. Depending on what it is you will need to treat the tank with medication but it would be good if you could show us a pic. That would help. The medication depends on what it is. Lets see what other people have to say.