Based in Al-Sadd, Doha, Moussa won the Arab junior decathlon title in 1998 and went on to finish twelfth with a personal best of 6872 points at the 1998 World Junior Championships in Athletics. He also won at the Gulf Cooperation Council Athletics Championships that year – the first of three straight victories from 1998 to 2002. He won the gold medal at the 1999 Asian Junior Athletics Championships, At the age of eighteen he was crowned the Arab champion in Beirut at the 1999 Arab Athletics Championships and improved his best total to 7482 points. He competed at the 2000 World Junior Championships in Athletics, only managing thirteenth place, and missed the 2001 season.