The word Rabb in Arabic means owner, master, coach and lord. If one says Imam Ali is Rabb meaning he is the ultimate Rabb, meaning the creator and Lord, that’s blasphemy and shirk, and we condemn that. But if the person says Imam Ali is my Rabb and he means my master, then that’s not blasphemy. The Prophet is our master, and the Imams are our masters too. But still one should not say such a statement because it could be misunderstood and it could cause confusion. So even if it has an acceptable linguistic meaning, one should be wise and not say something that could be misunderstood