What is the grammatical case of «مُوسَىٰ» in «رَأَيْتُ مُوسَىٰ»?

The Rule of the Muʿrab: Visible and Estimated Case

• Accusative with estimated fatḥa
• Nominative with visible ḍamma
• Genitive with visible kasra
• Indeclinable, in nominative position