Looks like a bacterial infection to me. Increase water changes and check your water perameters. Add 2 tbsp / 10g to the tank and get a good antibiotic medication, I like Maracyn 2 personally.
I would suspect poor water quality is the main contributing factor, but would you please give your water perameters? Ammonia, nitrite, nitrate, temperature are probably the most important. Would also give tank size and other inhabitants?
HTH