Is it true that magnesium is stored in hot water?

The above statement is false.
Magnesium is a reactive metal. But it doesn’t react with cold water like most metals. However, in hot water, it reacts to form magnesium hydroxide ($\mathit{Mg}(\mathit{OH}{)}_2$), which is insoluble in water and is slight of basic nature, and hydrogen gas. Also, it reacts with steam to give magnesium oxide ($\mathit{MgO}$) and hydrogen gas. Magnesium is naturally spread throughout the environment like rivers, seawater, and rainwater etc.
Reaction: $\mathit{Mg}+2H_{2}O\to \mathit{Mg}(\mathit{OH}{)}_2+H_2$
The corrected statement would be, “Magnesium is not stored in hot water”.