Large group of fungi that ferment sugars; yeast is a source of beta-glucan, which is a good antioxidant. The types of yeast used in cosmetic products are not harmful and in fact likely contribute to creating a healthier-looking skin surface due to their softening and conditioning action.
References for this information:
International Journal of Molecular Sciences, September 2015, pages 21,575-21,590
Annals of Surgery, November 1994, pages 601-609