The forms مُسْلِمَاتٌ and مُسْلِمَاتٍ display double-vowel endings called tanwīn.
For this noun class, ḍamma marks the nominative and kasra marks both accusative and genitive. With tanwīn, these appear doubled.
Compare definite forms: الْمُسْلِمَاتُ (nominative), الْمُسْلِمَاتِ (accusative), الْمُسْلِمَاتِ (genitive). Tanwīn disappears, but the case rule stays the same.