الْمُسْلِمِينَ can be either accusative or genitive without changing its written form:
رَأَيْتُ الْمُسْلِمِينَ.
Translation: I saw the Muslims. It is a direct object, hence accusative.
مَرَرْتُ بِالْمُسْلِمِينَ.
Translation: I passed by the Muslims. The preposition requires genitive.
Remember: Look at the verb and preposition, not only the final letters.
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Noun Case Classes: Type 6 — Sound Masculine Plural and Related Forms
الْمُسْلِمِينَ can be either accusative or genitive without changing its written form:
رَأَيْتُ الْمُسْلِمِينَ.
Translation: I saw the Muslims. It is a direct object, hence accusative.
مَرَرْتُ بِالْمُسْلِمِينَ.
Translation: I passed by the Muslims. The preposition requires genitive.
Remember: Look at the verb and preposition, not only the final letters.