Musa is Sham's nephew. He is the guy with the caravan that runs into Ali in Am Gezira, pleading for help with his broken down caravan. A plot constructed by Ali's mother Hatset to bring Ali back to Daevabad.
“Who am I?” the man asked. “Musa, of course!” When Ali narrowed his eyes, the other man feigned hurt. “Shams’s nephew? Cousin to Ta Khazak Ras on your mother’s maternal uncle’s side?”
(The Kingdom of Copper, p172)