Thank you for your question. While the previous prophets (as) all followed the essence of Islam, which is submission to Allah, they had their own divine laws and were not called Muslims. A Muslim is a term given to those people who follow the divine law as transmitted by the final prophet Muhammad (saw). For this reason, the believers who followed the previous prophets are not outwardly called Muslims, even if the essence of their religion is submission (islam) and the reality of the practice of that essence makes them one who has submitted (muslim).
May you always be successful