Call the root node $r$.  If you join each leaf node to a dummy sink node $t$, then $M$ would be an $(r,t)$ [*vertex separator*][1], also known as *vertex cut* or *separating set*.

Even without the dummy node, $M$ is an $(r,\ell)$ vertex separator for each leaf node $\ell$.

  [1]: https://en.wikipedia.org/wiki/Vertex_separator