That sure looks like columnaris to me- Flavobacterium columnare (F. columnare) Do some research into this disease. It presents in multiple ways. When it shows up as what you posted, it is often called cotton mouth disease. Another common one is white patches on the fish often at the base of the dorsal. In such cases it is often called saddleback disease. If it is columnaris, it is contagious and if left untreated will be fatal. Finally, there are multiple strains (aka genomovars ) of this disease. Some are so virulent they can wipe out a tank in a day before any external symptoms or visible. Other milder strains can take some time to kill and are easier to cure.
Treating columnaris requires antibiotics.
Treating columnaris requires antibiotics.
