My imam believes that there are awliya who can see stuff that humans cant see like jinns and angels and claim they posses the knowledge of ghayb , Allah has allowed maulanas to see such things and also there is a grave inside my masjid is it valid to pray behind this imam if i have no other options?

Ustadh AbdulAziz al-Haqqan

No, and don't pray in a masjid that has a grave in it. The salah is invalid and you need to repeat it as is the fatwa of the scholars amongst whom is Imam Ibn Baz.