Hey, I had an asian red tailed catfish that did that, I thought it was weird too, I honestly can't remember what answer was given for it when
I asked but he came out ok. I know the look you're talking about, I'm pretty sure it came from my water quality so you may want to do some tests if you haven't yet. I may be crazy here but something rings a bell about a growth spurt too.
I have a Striped Raphael Catfish for over 12 years now. I do notice some shedded skin coming off of its body once in awhile. It's definitely not slime coat for sure. I don't see any negative effect on the fish though. Well, at least it is still alive when its tank mates come and go