Mambo rafiki! The word "Watuzi" (it is also referred as "Watusi", "Watutsi") is formed by the prefix wa (plural), and tuzi (Tutsi), so the singular and plural forms for the Tutsie people would be Mtuzi, and Watuzi. The Swahili demonym is Mtutsi / Watutsi.
I hope it helps. :)