Judging by the reaction of Kalinda unable to tear her gaze away from the corpse's face, and Captain's use of the word traitorous, Zeph feels its a safe assumption that the rider is former crew of the Apparition.

The patterns of magic on his skin did bear a strange resemblance to how hull of the Apparition is woven together.... Throki could probably understand how the magic is interconnected...but he wouldn't have great expertise with how its incorporated into a person. Maybe Abu?

Given the reaction of the Captain, these are dangerous questions not to be asked on the deck of the ship. He'll wait for the meeting to share his meager conclusions.