According to my research…
[Scientific name] Senecio haworthii
[Japanese name / 和名] 銀月
[Common names] Woolly senecio / Ashweed
[Origin] South Africa
[Culture / Care] They do need some water during the summer, but not too wet. Fertilise annually, but lightly.
Can prune then back where the stem is firm, in early spring.
[Notes] Lots of website states that they like full sun and a very few watering. However, people actually reported that they do need some water and are not good at full sun and hotness in the Summer.
(備考) Full sunと書いてあるサイトも多いけれど、実際に育てている方たちのブログやSNS では、暑さに弱く、直射日光は避けるべき&水が好き、という記述が目立つ。日中、太陽が高く登ってからの直射日光は避けたほうがよいと思う。