Hi. It's hard to say what it could be. At first, I thought ick, but that would have gotten worse by now. Maybe bacteria or fungus? Do you add some aquarium salt to the tank? that always helps if you do after each water change. Also, try BettaFix. It seems to help out a lot of different diseases and since it is all nature, it will NOT harm a compltely healthy fish. It can't hurt. Keep us updated and I am sure some other people here will have better advise for you.