مُسْلِمُونَ is nominative with wāw, while مُسْلِمِينَ is accusative/genitive with yāʾ: this is the sound masculine plural.
By contrast, مَسَاجِدُ / مَسَاجِدَ shows vowel endings, with genitive fatḥa in the usual indefinite diptote context.
Different plural classes have different case-marking rules.
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Diptote Causes — Ultimate Broken-Plural Patterns
مُسْلِمُونَ is nominative with wāw, while مُسْلِمِينَ is accusative/genitive with yāʾ: this is the sound masculine plural.
By contrast, مَسَاجِدُ / مَسَاجِدَ shows vowel endings, with genitive fatḥa in the usual indefinite diptote context.
Different plural classes have different case-marking rules.