i wouldn't add salt, that is usually effective for ich treatment, which this is not. just continue with the larger, more frequent water changes for now and monitor the betta, as well as the new fish, for any additional 'white stuff' or any other symptoms.
still would like to know how long this tank has been set up, and the water parameters (ammonia, nitrite, nitrate).
still would like to know how long this tank has been set up, and the water parameters (ammonia, nitrite, nitrate).