The alien's true appearance will slowly be unveiled throughout the course of the next ten episodes, which of course USA Today completely ruined by revealing the above photo earlier today.
Producer Scott Rosenbaum explained that he needed to flesh out the backstory of the visitors before he and his team locked down their true guise, "There is a reason the creature I ended up wanting to design looks familiar in many ways and unfamiliar in many ways. Throughout the season, you'll be seeing exactly what the Visitors look like underneath, and while some of it will be explained as to why they look the way they do ... if you look close enough and study it, you can sort of figure out what the Visitors have been doing throughout the course of history in the universe. Answers to that are revealed literally in (their) physical nature."
Source: MovieWeb, USA Today