How is mercury different from other metallic elements?
It's the only metalic element that is a liquid at freezing point (0 Celsius).
There are only two elements that are liquids at room temperature, the other one being bromine, a non-metal.
So yes, mercury is unique in that way (melting point around -39 Celsius). Nearest contender is Gallium, that litterally melts in your hand (MP=29.8 Celsius)