Can it be that Merus is the grand priest? Here I saw this line. 
 "However, Merus tells him that his doubts are now gone and asks Goku to follow him as he heads inside a nearby pyramid. Behind the door lies the Galactic Patrol's Hyperbolic Time Chamber. Merus tells Goku that every day that passes in the real world amounts to three days in this room, but the main reason for heading inside is so that he can unleash his full power without anyone finding out about it." 
which made me to wonder whether Merus can be the grand priest? (BTW Merus's eye shape matches to that of grand priest )
From Chapter 54 of the manga it turns out that Merus is not grand priest but rather an angel.