When I first got Sid from the pet store, his poop looked exactly like this.
I really thought he had parasites, but as the recommended treatment wasn't anything I could easily run out and grab, being at the time in hospital-at-home care, it had to wait.
Turns out it wasn't worms. I still don't know what it was, but it's for the most part gone now. He still gets the occasional stretch of white in his poo, but the rest looks normal, and the white is less frequent now.
Sid was very thin when I got him, and lethargic as heck compared to what he's like now. I think regular feeding with high quality foods plus brine shrimp and mosquito larvae might have helped.
I can't say if it's the same thing Sid had, but it looks exactly the same in the pictures. I couldn't find anything like it online, either. Maybe a gut infection? The result of long-term starvation in the pet store?
I know worms don't spontaneously go away in mammals, not sure about fish, but his increasing levels of health and activity, and the normalising of his poo strongly indicates to me that worms were never the issue.