It is very difficult to tell what exactly it is from the picture, but based on your description, it almost sounds like heximita. Can you please list your water params for ammonia, nitrite, nitrate, and pH? Also, size of tank, number and species of fish, feeding and water change schedules, type of filter, water temp, and any other information you can give about the tank?
I will help all I can. Heximita is treatable if it's caught soon enough, but we need to make sure we know what we're treating for and check water params before adding any kind of meds to the tank. The wrong meds or poor water quality can just as easily wipe out a tank as what the disease can. Please be careful.