I think I'm seeing that in the last pic you posted
OK, so we have the telltale stringy white poop--plain as day in the pic even. Strong indicator of "parasites". So, what kind? Protozoan, bacterial, worms, or flukes? I think the most likely "parasite" causing this is protozoan or worms.
We prolly don't have the equipment, skill, and knowledge to take a stool sample and identify the "parasite" under a microscope, so we take a guess. Of the 2 -- protozoan or worms -- possibilities, one choice has an easy, cheap, and non-toxic treatment. That's Hex.
Hex is very common, and treated with epsom salt, which is non-toxic to the fish. So, before treating for worms, I'd try the Hex treatment and see if the fish stops the stringy white poop. If it does, we can be almost certain that it was Hex because it responded to our treatment.
RD wrote a sticky in the diseases forum that tells exactly how to treat with epsom salt for Hex. I've done it myself and it worked for my fish in 3 days.
I'd isolate the fish in the tank with a divider. Makes it easier to see that he's eating medicated food. Easier to net him if you have to get serious and shoot the solution down his throat. Easier to watch his poops if he's isolated, too.