Initially I didn't realize that your male Eclectus was indeed a plucker, so I had other ideas as to what might be going-on with the yellow feather(s); however, now knowing he's a plucker probably answers the question...
How long have you had Chuck? Was Chuck plucked when you brought him home, or did he only start plucking since you've had him? Also, an important question that hasn't been asked (though it can probably be assumed) is what sub-species of Eclectus is Chuck? (I believe there are around 10 sub-species of Eclectus, if I remember correctly,
though it's been a long time so correct me if I'm wrong on this).
It's probable that the stray yellow feather or feathers on Chuck are due to damage to his feather-follicles that he's done while plucking; however, it's also possible that they are due to Liver Disease, or with an Eclectus it may also be due to a dietary allergy/sensitivity. The bottom-line is that since Chuck is an Eclectus who is a plucker, he needs to see a Certified Avian Vet or Avian Specialist Vet who has experience and knowledge of Ekkies, as many CAV's don't have much experience or knowledge of them unless they have treated them...It's very important that any parrot who plucks, or any parrot at all, but more so if they pluck, has routine, Baseline Blood-Work done at least once a year, if not every 6 months, so that the levels/results can be compared each time they are taken. Routine blood-work will tell you if he's suffering from any Liver or Kidney disease, infections, anemia, metabolic issues, dietary deficiencies, etc. This is important for all parrot owners to know, but especially an Eclectus, and especially one who is plucking...