The truth about the White Ghost and why Kensi was sent to Afghanistan to hunt him down were both revealed in NCIS: Los Angeles Season 5 Episode 19.
Once again, Hetty kept secrets and it ended up to the detriment of her agent. When will she learn that sometimes knowledge is much more important than secrecy? She's repeatedly put her agents in harm's way without providing them important details about their mission.
The White Ghost that Kensi was sent to kill never existed; instead, the CIA pegged Jack Simon as the White Ghost as a cover to kill him.
Jack was working for them and knew many of the CIA assets in the area, but when he decided to stay and live among the locals he became a liability.