Continuous growing wing feather

(Canada)

Our pet Blue Fronted Amazon has a secondary covert wing feather that is growing continuously. It now extends well beyond the end of the primary feathers. See attached picture. This feather is also limp compared to a normal feather. What are the possible causes of this and will pulling the feather correct the problem? The bird is 8 years old and is one of our "own" offspring.

Our pet became sick about a year ago. He stopped vocalizing & chewing wood blocks, had panic attacks and some seizure-like episodes. Our vet was not able to determine the cause of his problems. We eventually discovered on our own that yougurt sweetened with splenda was causing his problems. The bird has fully recovered since we stopped giving him the sweetened yogurt. Milk Thistle powder was given to help detoxify his liver from the effects of splenda and his overall behavior has now returned to normal.

I'm a retired parrot breeder with many years of experience and am at loss to determine why his one wing feather is growing continuously.