This is a property of $\mu$, not that of $\mathcal A$, and it is called being atomless. It is equivalent to not having sets $A \in \mathcal A$ of positive measure such that for all $B \in \mathcal A$, $B \subseteq A$ the measure $\mu(B)$ is either 0 or $\mu(A)$.

edit: [Wikipedia article][1], complete with the proof of the property you describe from atomlessness.


  [1]: http://en.wikipedia.org/wiki/Atom_%2528measure_theory%2529