According to your linked picture, the area you have that's unexplored is cut off by an instance barrier. You can get access to it by accepting the quest [HEROIC 4] Resistance Sympathizers which can be accepted at the bounty board at the Imperial base in the Sundari Flatlands.
^ This kind of stuff is a problem when trying to do codex/achievement stuff. Everything should be findable without having to do some Heroic and have to go online and search for 30 min to find out what to do to get it. It should be right there. Drive up and get it, done.