If the white stuff is not fluffy or particularly fluffy in appearance then it is columnaris, you need a med that treats both fungal and bacterial infections to get rid of this.
Columnaris most commonly comes about when the fish are put under some sort of stress, like the tank/water quality is not kept clean enough, tank is over-stocked or over-crowded, fish are harrrassing each other because they are unsuitable tankmates etc.
Siamese fighting fish/Betta's are primarily solitary fish by nature and tend to thrive best when kept on their own. The best thing you can do to treat the fish right now is to get it a tank of at least 2gallons and acclimatise the betta to it. Do a 20% water change on the tank once a day with Stress Coat or AquaSafe dechlorinator while treating the tank with an anti fungal and bacterial medicine like Pimafix- increasing the airation of the tank will also help, either by adding a filter or bubble pump, but make sure the current in the tank is not too strong (betta's/siamese fighters don't like strongly filtered tanks, in the wild they originate from slow moving waters like ponds in Thailand).