They may have a similar stature to Sahagin, but the Mamoolj'aa share more physical similarities to that of the Amalj'aa which would indicate a stronger evolutionary connection.
Look at their head crests for instance: While the Mamoolj'aa have one that is pointing straight upward, the Amalj'aa have a massive bulky one (like the rest of their musclebound bodies) that falls behind the head and back.
Maybe the Mamoolj'aa left for La Noscea because they got sick of getting bullied by the big meathead Amalj'aa and having sand kicked in their faces.
