As everybody knows that reciting 'Moulood' (reciting some Arabic script on so called Prophet Mohamed birthday) is an 'Bid'ah' innovation. Can we pray behind the people who practices Bid'ah.

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions. To know the rulings of Shariah about celebrating Prophet's (Sallallahu Alaihi wa Sallam) birthday please read the Fatwa:  81765. Therefore, the rulings of such gatherings apply to the hymns that are recited in such gatherings. However, if some good poetry or hymns are recited out of such gatherings and do not contain any prohibition such as polytheism, innovation, music, etc. then that is permissible. As for praying behind an Imam who practices Bid'ah, please read the Fatwa: 82606. Allah knows best.