Question: are you saying that this photo is AFTER a trim? Is your horse a barefoot horse? If so, I would say those walls have been left a bit too long. Of course it's hard to tell just from one photo. It looks like a "pasture trim" to me, which most or at least some "regular" farriers will do. A "barefoot" trimmer would take the walls down to nearly the level of the sole, if not all the way to the sole. Maybe it's just a stretched white line from walls that are left too long? That is my guess. A pic of the hoof taken from the heel forward while holding the foot up and a pic of hoof on the ground would help to see if the walls are too long and the hooves are flaring at the bottom. That would indicate a stretched white line. Maybe i'm crazy but again, hard to be sure with just this one photo. Otherwise, those are nice hooves!