Mūsā as the object

The Rule of the Muʿrab: Visible and Estimated Case

رَأَيْتُ مُوسَىٰ
Translation: I saw Mūsā.

رَأَيْتُ: past verb with the attached subject tāʾ.
مُوسَىٰ: direct object, accusative, with an estimated fatḥa.

Compare
رَأَيْتُ زَيْدًا — I saw Zayd; overt fatḥa.
رَأَيْتُ مُوسَىٰ — I saw Mūsā; estimated fatḥa.

Both nouns are direct objects. Their syntactic role agrees; only how the ending is expressed differs.