رَعْيًا
Translation: May you be cared for and protected!
رَعْيًا: accusative maṣdar governed by an understood verb.
Expanded: رَعَاكَ اللَّهُ رَعْيًا; raʿā is the verb, -ka its object, Allah its subject, raʿyan its absolute object.
The omission is tied to conventional usage, not a universal permission to drop verbs.