Mamaati, it comes from the Arabic word Mamaat, which means death. It is a big discusion amongst the Ulamaa of Islam whether Rasoolullah Sallallahu alaihi wasallam is alive in his grave or not. Those who consider him to be dead in his grave are known as the Mamati group.

If the Imam is a Mamati, and his other views are sound, and he is not from the people of Bid'ah, then to pray Salaah behind him is permissible. It would be best to avoid it if possible to avoid confusion.

And Allah knows best.