Microsporidian, family Nosematidae. Intracellular obligate parasite. This parasite is intracellular, and they typically infect any large organ of the fish, after the fish eats a spore. It replicates within the organ(s) of the fish which eventually become cysts, and can often be observed as white nodules on the skin of the fish or tail. They eventually burst and release numerous spores into the water and the cycle begins over again.
No known cure. Invariably fatal.
From Dis.Aquat. Org. Vol: 125-129 Cecile Pomport-Castillon et al.