Hydrangea luteovenosa Koidz.

Family: Hydrangeaceae
Japanese: コガクウツギ (小額空木) [Kogaku-utsugi]

A small hydrangea with white flowers and small, jagged leaves that I would say have a bluish tinge. It's the first hydrangea to bloom here and is often seen clinging to the wall-like rocks along the trails..

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）