Is my prayer valid behind an Imam who says "Salam alaykum wa rahmatullah" instead of "As-salamu alaykum wa rahmatullah" when making the taslim, and what is the ruling on my prayer behind him if he is unaware of this?

The taslim (definition) (salutation) is one of the pillars of prayer according to the majority of scholars. It is a condition that "al-" (the definite article) be present in "al-salamu 'alaykum" (peace be upon you), and "salamun 'alaykum" (peace be upon you, without "al-") is not valid. The Hanafis differed, viewing it as an obligation, and that prayer is valid with "salamun 'alaykum," but they consider "al-salamu 'alaykum" to be more complete. If the Imam follows the Hanafi madhhab (definition), prayer behind him is valid, with the recommendation to advise him to use "al-" to avoid disagreement. However, if he is not Hanafi, his prayer is not valid, and he must be advised.