There are many conditions that present these symptoms.
Saprolegnia, Lymphocystis, and Flexibacter columnarae are the most common.
If the mouth starts to protrude, it may be Flexibacter, lowering temp can slow the disease, but it is usually fatal.
If Lymphocystis, it is viral (like the common cold) and must run its course and be controlled by the fishes immune system.