Rechargeable Mg-metal batteries (RMBs) are considered promising alternatives to conventional Li-ion batteries owing to their high volumetric capacity and low cost. In addition, Mg anodes for RMBs do not suffer from metal dendritic growth or internal short circuit. However, the notion that Mg anodes are indeed dendrite-free has recently been under debate, and further clarification is crucial for advancing practical RMBs. In this work, we closely investigated Mg dendrite behaviors under various electrochemical test conditions using operando observation techniques. The critical current density inducing fatal Mg dendritic growth was defined by directly monitoring the dendritic growth process leading to a short circuit. We further propose a new strategy to regulate the dendrite growth by introducing magnesiophilic sites of Au nanoseeds on a substrate. We not only elucidated the effect of the applied current density and capacity utilization on the Mg growth behaviors but also demonstrated the effect of magnesiophilic seeds in suppressing Mg dendrite growth.