How do I access this location under the bridge in Dark Souls 3?

It's a bit hard to tell from the picture alone but that seems to be the cave Midir falls in after you attack him enough when he's perched on the bridge. That said, I don't remember an item there so it might just be a light effect.

UPDATE I first mistook the floor where the item is for water and though it was

Midir's boss room

It's actually a small cavern you can access right after the lower bridge. Starting from the Ringed City Streets bonfire, head out toward the Shared Grave, cross the bridge then hug the right wall. You'll soon reach a small hole that leads to the cavern seen in OP's picture. The item is some titanite scales.