wtf is that arrow?

I'm not sure, but I think it's pointing to another set of hands.

That is not an arrow, it's 'n~', which is kind of like a moan here.

Admittedly, pointing out the んーー was my first reaction too, but there's also an arrow to the right of Reimu. The pixiv description and tags didn't shed any light on what it's pointing towards.

I totally didn't see that arrow.

Marisa's probably fighting Remilia for possession of Reimu.