Mikazuki / 三日月: “Crescent Moon”. This will sometimes be romanized as “mikadzuki” as tsuki / つき / 月 becomes (d)zuki / づき because of rendaku.
Oku Hanako appeared with Ariyasu Momoka / 有安杏果 (Momoiro Clover Z’s green member) on the October 17, 2015 episode of Atashi no Ongaku / あたしの音楽 (“My Music”)[jp]. Together, with Oku-san on the piano, they sang a duet cover of Ayaka‘s Mikazuki.
It’s very beautiful! I actually prefer this to the original. They’re both strong singers and they complement each other.
It reminds me a little of something she might have done in her early years. Comparing this with her early works, though, I can hear that Oku Hanako has definitely grown in her singing abilities. Back then, she would not have been able to hit that high note in the chorus so cleanly. She also wouldn’t have added the vibrato present in a few places.
This song is a nice break from her most recent works. There was always something appealing in the simplicity of a lone piano and I hope she does more of it.