@JeremyECrawford Does it take an action for a Ghost to shift from the Ethereal to the Material Plane and vice versa?
— Hanno Ziegler (@ronin_sama) January 10, 2016
An ability under a monster's "Actions" heading is an action. E.g., a ghost's Etherealness is an action. #DnD https://t.co/TLiq6xzbSC
— Jeremy Crawford (@JeremyECrawford) January 11, 2016