Mawlā here is not the usual social term for a master or patron. The expression connects it with land affected by walī, a subsequent seasonal rain that promotes fresh growth.
Al-usrah relates to a valley or a local designation in this expression, while aghyad suggests supple, gently swaying growth. The governing image is of soft pasture after seasonal rain, nourishing the she-camel.
A polysemous word must be read in context: importing the common social meaning of mawlā here would make the landscape description incoherent.