I don't want to alarm you but I think that might be the onset of chronic columnaris but I'm not sure since I don't have any experience with it.. I just know the symptoms and such in theory.. Usually the area on the back loses colour and then slowly gets cottony (its called a saddleback).. I got this rom Darkmoon's Diseases and Cures sticky, I hope it helps:
•Symptoms: White spots on mouth, edges of scales and fins, Cottony Growth that eats away at the mouth, Fins rapidly disingrate, starting at the edges
Gray areas around head and gills, As the disease progresses the gray lesions may change in color to yellow/brown/red, Lesions often occur in front of the dorsal causing a “saddleback” appearance, Lethargic, Loss of appetite, Clamped, Gasping for air
•Treatment: There are 2 versions of Columnaris: chronic and acute. Chronic Columnaris can take days to progress while acute can kill within a day. It is contagious so isolate sick fish. If more than one fish shows symptoms then treat the entire tank. Perform daily 100% water change in small tanks or ¾ water change in larger tanks. Make sure to clean the gravel. Treat with Aq.Salt: add 1 tsp/gal Aquarium Salt 3 times, 12 hours apart so that you end up with 3 times the normal concentration. Do NOT raise the temperature as it thrives in temps over 85*F, however, lowering the temperature does not seem to help fight it. Combine salt treatment with Mardel’s Coppersafe, Maracyn I & II, API Erythromycin, OR API Triple Sulfa, combined with Jungle’s Fungus Eliminator (if possible).