Could be Slime Disease
This disease is also caused by protozoa e.g. Chilodonella, Ichthyobodo (Costia), Trichodina and Brooklynella species, and Trematodes (flukes) e.g. Gyrodactylus and Dactylogyrus species.
These parasites spend the whole of their life cycle on the fish, although infective young parasites may be released into the water to spread to new hosts. The parasites live on the skin and gill surface, eating gill and skin tissue debris. If their numbers increase rapidly, they may irritate the skin and gills, causing excessive mucus production and usually killing the fish by smothering the gills and hence suffocation.
But
Up the temp on the tank with the salt Methylene Blue cures it but thats old school there are other things out there now.