It looks to me like an old abscess blowout. Judging from how far down from the hairline it is, it probably happened at least a couple of months ago. I wouldn't worry about it, but you might talk with your farrier about keeping it trimmed to not overload the section of hoof directly below it. If it bears too much weight, it is entirely possible that a large section of the hoof wall, where it has detached around the blowout, could break off, leaving a gap in the weight bearing part of the wall.
Even if that were to happen, it's unlikely that it would cause problems, it would just look terrible until the broken section grew out.