Melafix is best for healing damaged fins and should NOT be used in a full dosage for bettas. It will also not cure fungus if that's what this is.
If the area is cotton wool looking/fluffy then this would denote a fungal infection. This is often highly contagious so you should separate the betta into his own tank.
A large critter keeper with a sponge filter and heater, plus a coconut cave to hide in and some silk plants will work great as a hospital tank.
A good treatment ( if in the UK ) to use for fungus is Myaxzin. Again, a half dose is best for bettas to be on the safe side, as they can be sensitive to some meds.