He’s been in QT for 1 week and 2 days in a 30gal with cupramine dosing of 20gallons what is this on his side? He is also pooping white string however all other fish in quarantine seem to be doing fantastic.
Bacterial is definitely a possibility. It could also be Uronema. I would treat the fish in a QT with a wide spectrum antibiotic that includes metronidazole in case it is Uronema. The problem is if it is Uronema all of the fish in the tank would need to be treated and there is not a fallow period that can erradicate Uronema because it has a direct life cycle and can live without the presence of fish.
FWIW I have seen alot or Uronema, the only fish it has ever afflicted in my care are chromis/damsel family, Wrasse, and dwarf angels. So many fish can live in the presence of Uronema and not be impacted.