That looks like it's what's left from an old injury or such. It could be just a scar line which will always stay. If it's just a groove(hard to tell) I wouldn't worry about it. If it's actually a crack, which it appears it could be but doesn't look deep at all, there might be a bit of infection in there perpetuating it. In that case, paring it out(just only so far in as it needs) and/or treating the infection by soaking in a solution of copper sulphate or such will hopefully be all that's needed.
The other minor cracks, past the half way mark on this hoof are what dietary/nutritional issues can look like. If the top of the hoof is anything to go by, it seems that she's been on a better diet for the last few months at least.
There are some imbalances/flaring in that hoof, that it would be helpful to address too, although I guess they're not effecting the crack at present. If you'd like a general critique on the horse's feet, more info & more pics would be good. See Good Hoof Photos - How to take Good Hoof Photos
to learn about good critique pix