Batteries based on Mg metal anodes promise high capacities and dendrite‐free metal deposition, making them one of the most promising candidates for post‐Li(Na) energy‐storage technologies. Herein, for the first time, the fundamental issue of solid electrolyte interphase (SEI) growth under open circuit voltage (OCV) conditions in a Mg salt–triglyme electrolyte as observed by electrochemical impedance spectroscopy (EIS) is dealt with. The EIS suggests that SEIs are liquid/solid composites where ion transport occurs predominantly via diffusion in the encapsulated liquid. Depending on the salt choice, volume percent of the liquid part of SEI is varied up to one order of magnitude. The surface limiting reaction growth mechanism proceeds through SEI densification or as a consequence of inherent mechanical instabilities.