All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.
If a person recites "mal-hamdulillahi Rabil-ʻAalameen" in Soorah Al-Faatihah in prayer, his prayer is invalid, because this is a significant mistake that alters the meaning with negation, since "ma" is an article of negation (it negates what comes after it) in the Arabic language. So, if recited in this manner, it negates that praise is due to Allah.
But if he recites it as follows: "mil-hamdulillahi Rabil-ʻAalameen", it is also a mistake and an addition of a letter at the beginning of Al-Faatihah; however, we believe that this mistake does not alter or negate the meaning. Accordingly, it does not invalidate the prayer.
Such a person should not be an imam in the prayer.
For more benefit, please refer to fatwa 343006.
Allah knows best.