Hi you can not be sure but it sounds like your betta may have White Slime Disorder
Stringy, white mucus on the body, highly contagious.
Treat the fish with an antifungal, treat fish he has had contact with, and scrub, scrub, scrub the tank or bowl with bleach. If he's been in a community aquarium, I'm sorry, but you're going to have to dismantle the entire thing, sterilize it, and recycle it before putting the fish back in. Yes, it is THAT bad.
However seek others opinions a picture of the ill fish might help diagnosis.
Dont be alarmed I may be wrong but hope this post helps you towards finding the problem