clear and stringy like could be internal parasites. And it better to treat fish right away because they get worse really fast. I would keep him on Epsom salt 3tsp/gall with daily 100% water changes. Buy medications and use it along with salt.
I know someone treated with Parasite Guard Tetra, Jungle Parasite Clear,
API General Cure ,Jungle Labs Parasite fizzy tabs .
Epsom salt don't worry you can use for long time and it will help /should help to pass the worms out the system. But IT really helps if the fish is eating so he can produce waste and pass worm with poo.