«جَاءَنِي الْقَوْمُ مَا خَلَا زَيْدًا»
mā: maṣdariyyah; khalā: fixed past verb in this construction; Zaydan: accusative direct object.
Mā ʿadā follows the same accusative pattern. With this mā construction, do not make the following noun genitive.
The source places mā khalā/mā ʿadā under the accusative rule.