يَا عَبْدَ اللَّهِ
Translation: O servant of Allah!
يَا: vocative particle.
عَبْدَ: accusative vocative in iḍāfa, first term of construct.
اللَّهِ: genitive second term.
An iḍāfa vocative is overtly accusative; a simple proper-name vocative such as “yā Zaydu” follows a different rule discussed next.