No, Ben Mendelsohn is not playing a Grand Admiral in Rogue One: A Star Wars Story.
As part of EW's reveal of the full character list for the upcoming film, the first of the standalone live-action movies that don't fit in the episodic structure, Mendelsohn's mysterious white-caped character has been revealed: Director Orson Krennic. There was much speculation that he'd be a canonized Grand Admiral, but his title, while just as rare, is not entirely new.
The "Director" title was introduced in two of the new canon novels published since Disney purchased Lucasfilm and rebooted the expanded universe. "Directors" can be found in the pages of Tarkin and A New Dawn, and are typically associated with one particular section of the Empire, like the ISB, the Imperial Security Bureau (or, essentially, the secret police of the Galactic Empire). That fits with the other guesses about this character, thanks to his white clothing, that he'd be associated with that agency.
Director Krennic is a "really smart, and really effective" villain in the Empire's ranks, and the head of the Deathtroopers, the elite special forces unit of the Imperial military (those are the new black armored Stormtroopers seen in the trailers). His desire to rise in the ranks of the Empire at all costs is indicative of the nature of the entire Empire. "There is a lot of palace intrigue going on in the Empire, with people conspiring to move up the ranks and sabotaging each other. There’s not a lot of loyalty there," executive producer John Knoll revealed.
There may be more about Director Krennic revealed soon, and if not today then almost certainly when Star Wars Celebration Europe rolls around in July.
Rogue One: A Star Wars Story hits theaters December 16, 2016.