The generic term is "fin rot", it is usually bacterial, so an antibiotic, and/or some sort of general cure.
Fin rot is usually a sign that not enough water is being changed on a regular basis.
What size tank?
and what is your normal frequency and what volume of water are you changing?
Also how often do you clean out filters?
And what is your normal nitrate reading in ppm before a water change?
These are all significant to the "why" it may be present in the tank.
Does look like fin rot. Salt and water changes maybe enough to get it better. It looks mild and can get better w/o an abx. Make sure its getting clean water. Need to answer the questions Duane’s asked.
Fin rot is a generic name for a bacterial infection, it can occur anywhere on the body the bacteria can take hold, fins, eyes, on the body. It often starts on the fins, because they are easily damaged.
In a quarantine tank, I tend to change about 50%of the water daily.