See the white cotton like spots in the picture. Ich (Ichthyophthirius) is a pesky little parasite. If you always add aquarium salt to your betta’s water (1 teaspoon of aquarium salt per 2 1/2 Gal of water) and one drop of Aquarisol per gal, your betta will never get ick. It is very contagious, but bettas will fully recover if treated promptly. Frozen live food may carry ich.
Small white spots resembling cotton / sand
Fish scratch against rocks and gravel
In advanced stages fish become lethargic
Redness or bloody streaks in advanced stages
Ick is a common disease and is very contagious, since its contagious, once you see symptoms of that disease on any fish, immediately do a full water change and start the treatment. It is preferable to treat the whole tank when one fish is found to have it. Ick is temperature sensitive and hence raise temperature to 85 F and add one drop of Aquarisol per gal every day until cured. It will only take a few days to get rid of the pesky little parasites. Do a full water change and add one drop of Aquarisol and salt to the water. If you have a tank and heater, raise temperature of water to 85 F. Ick are sensitive to heat and will detach from fish at higher temperature, at the same time Aquarisol medicine abosrbs ick and kills it. BettaZing can also do the job, in case you do not have aquarisol on hand.