# What is the compound in MgS?

Nothing. $\text{MgS}$ is the compound. Any compound is a combination of two or more different elements. And naturally, $\text{Mg}$ must be different from $\text{S}$.
Of course, if the compound is $\text{MgS}$, which contains two atoms, there couldn't be any "sub-compounds" within this compound, even if that were a thing, because the only atomic-scale components smaller than a diatomic compound (besides what's in an atom) is an atom.