Slime coat deterioration???

One of my bettas has something I've never seen before. I think it could be some sort of slime coat issue. His scales are going white - almost looks like flaky, dry skin. He's in a 5 gallon filtered, heated (78 degrees) tank and I do water changes every 1-2 weeks. I'm using API Stress Coat and Aquarium salt but the scales seem to get worse by the day. I'm concerned that if this is a slime coat issue, he's going to become more susceptible to a whole other host of problems, which is already evident in his abdomen.
If anyone has seen or dealt with this before, your advice would be appreciated.
I've attached a few photos.
Thanks