That was bacterial.. I'm sorry :(
Just for future note, bettas need to be kept in 2 gallons minimum with a reliable heater that will keep the temp between 78-80F without any swings. An in tank thermometer should monitor temp. In this size twice weekly water changes are needed of 50% and 100%. Conditioner (preferably liquid) which remove chlorine, chloramines and heavy metals is a must. They should be getting two small meals a day, at least a couple at a time and one fast day a week. A pellet should have the first two or three ingredients are whole fish and not fish meal or wheat.