That crescent is the lunula. We don’t all have them (so don’t panic if you don’t!). The Oxford English Dictionary (OED) carries a September 1891 listing from the British Medical Journal ...

It is used to treat mild to moderate fungal nail infections that does not involve the lunula, by painting it onto the infected fingernails or toenails. The lunula is the crescent-shaped area ...

P. lunula cannot use that same strategy: it has chloroplasts organized in the form of a complex network, necessitating a different form of motion to avoid bright light.