From the photo, the problem looks to me like Pleistophora hyphessobryconis or similar, a sporozoan disease which is reasonably common in tetras. (Sporozoa are classified as protozoa, like white spot, but are much smaller and very different in their disease-causing behaviour. If you can Google it, images could help you to ID it.) The disease is internal, affecting muscle tissue (turning it white). If that is what it is, there is unfortunately no cure that I am aware of. The disease, if I have identified it correctly, will be relatively slow to develop. (How long has the fish had it?) Affected fish can appear behaviourally unaffected for some time, even when the disease is quite advanced. If you are able to isolate the fish, this would reduce the chances of the disease spreading, though in my experience it is highly unlikely to become epidemic in an aquarium, which is an upside. The mechanism(s) of spread is/are poorly understood, but some schools of thought believe fish consuming an infected dead fish is likely to be one.
If you decide to remove the affected fish, the easiest way in a planted tank will be at night after the fish have well and truly 'gone to sleep'. In the dark, use a torch (flashlight), moving it slowly towards the tank to avoid startling the inmates, and catch the fish out. Very much easier than doing it while fish are awake!!!
Nice looking tank by the way, with the plants. (Love the barbs and hatchets too!)