مُسْلِمِيَّ may look shorter than a regular plural, but it remains a sound masculine plural changed by iḍāfa and assimilation.
Do not automatically read this form as a singular or infer genitive merely from kasra. Determine its case from the full sentence.
Test: in «جَاءَنِي مُسْلِمِيَّ», the plural is a nominative subject.