Dark Souls Remastered | Crystal Cave "hidden" item on distant ledge, how to get it & return

The item is a Blue Titanite Slab.

The orange lines on the ground are messages left by other players to help show a path. If you're not lucky enough to have those, you can look closely at the snow as it falls, which will tell you where a ledge is. If that's not good enough, you can also use Prism Stones. Right before leaving Duke's Archives, there's a chest with 20x Prism Stone. You can also buy Prism Stones at a vendor.