Maesa japonica (Thunb.) Moritzi et Zoll.

Family: Primulaceae
Japanese: イズセンリョウ (伊豆千両) [Izu Senryō]

While there are two Maesa species in Yakushima and both bare white berries, M. japonica is in general a much smaller, less conspicuous plant and the berries are mostly covered by a whitish calyx.

References:
九州野山の花 片野田逸朗,2004：南方新社
米倉浩司・梶田忠 (2003-)　「BG Plants 和名－学名インデックス」（YList），http://ylist.info（ 2019年8月14日）