Its the quest which will most likely continue the story, you have to remember they already planned to do so much and its very likely they already added that in but didnt have enough time to actually finish the extended story so left the monitor the last quest but kept the following quest challenge still visible as ??? ,its much like when the game released and you completed the story you had quest markers to speak to people to carry it on yet you couldnt actually interact with them and so my guess would be its a undo able quest atm. i would say it could be complete all other challenges yet you cant without doing that one so it cant be that sadly.
This has been identified.
It is believed to be related to the 'Legendary Freelancer' challenge that wasn't completing, a bug addressed in patch 1.1.1.
Sadly the 'fix' isn't retrospective.
To resolve it either:
a) be lucky to run the 'Return to the Cenotaph' mission from quickplay, completing the critical path storyline.
b) start a new pilot and run through the critical path, again to the point the Monitor is defeated.
Either should complete and resolve the '???' issue. (worked for me)