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