If the said imam has shikrkiya belief then you should not offer salah behind him. And if he does not have such beliefs rather is involved only in bid’ah then it is better to offer salah behind him instead of offering salah alone so that one can get the virtues of Jamah.
صلى خلف فاسق أو مبتدع نال فضل الجماعة. (در مختار)
What does the said imam says to the people to practice on the grave of the auliya? It is not proved to offer the said Salam at the grave of any person. It is innovation and bid’ah.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband