Mg metal batteries have attracted much attention as an alternative to Li-ion technology due to the high abundance and volumetric capacity of Mg metal. Further, early reports show that Mg is less prone to dendritic growth compared to Li, thereby improving the safety and long-term reversibility of Mg metal anodes. However, dendritic growth of Mg can be observed in various conditions, causing cell shorting and capacity loss. Herein, we report a chemically-formed Mg-In alloy interphase that suppresses nonuniform Mg growth during electrochemical reduction. Ex-situ X-ray diffraction shows that upon reduction, Mg alloys into the Mg-In interphase with no evidence of Mg deposition on top of the surface during initial cycles. Interestingly, further reduction results in Mg depositing underneath the interphase, which confirms Mg mobility through the interphase. However, the alloying reaction is kinetically limited, leading to significant Mg deposition on top of the interphase at high current densities. Thus, alloys on Mg can affect deposition morphologies, but are limited by the kinetics of Mg conduction through the alloy.