The music Kubo plays attracts something's attention.

It watches them from the trees. And the ground. It watches them from under the mud. It's everywhere and therefore it can watch them from everywhere.

Suddenly, something starts to rise out of the mud in front of them, a creature made of vines and leaves and algae and swamp muck. It forms itself from the living plant matter of the swamp, and stares them down.

It doesn't attack but they have to decide whether or not to attack it.