I'm 99.9 % sure that Bandit is correct. Your Denison Barb is suffering from Columnaris. Five years ago I lost 3 out of 6 Denisons to the same disease. All were full grown adults, 4 y.o.
5" long. One started have a spinal bend like fish TB. All three had the white growth around the mouth. Once I recognized the disease, I removed and euthanized the sick fish. Afterward I conducted a world class gravel vac / WC
. All other tankmates survived. Only the Denisons were affected.
, once the symptoms of Columnaris are noticed, meds are useless. In my case: although I always practice regular water maintenence, I tend to overcrowd my tanks. That situation is not compatable with Denison Barbs. They are a fish that require pristine water and evidently are susceptible to bacterial infections.