You have a 10g unfiltered, heated(70-75F) tank with 3 Bettas (divided) making weekly 50% water changes for the past 6 months and now they have what appears to be white flaky substance on them and the water is cloudy-fish continue to eat and act normally
How much is a little salt?
I would start by getting your temp stable at 76-77F, start making daily 50-75% water changes and using a bit more dechlorinator and no salt
Columnaris is another thought-google that and see if it looks like what they have
Really need a pic-
most likely what you are seeing is excessive slim coat and that can be caused by some type of irritant from something in the water or a parasite
How are they looking and acting today