This sketch is part of the Buddha Nature theme.
It requires quarter tones and I still haven’t decided which notation style to use. So I’ve tried a couple in the above page: both the arrow on its own as well as a natural sign with attached arrow mean the same thing.
I personally prefer the former but the latter is more conventional. However I don’t like the accidental for 'quarter-tone flat' because it is a flat sign with an arrow pointing UP which is counterintuitive for a flat note. So I avoid it and I will use an enharmonic quarter tone accidental to replace it.