Hi, That's an old abscess or 'fault' crack, that was due to a minor injury on the coronary band & has been growing down for the last few months. Appears he could have had periople(cuticle, the dirtier looking ring above) covering it until recently, why you haven't noticed it before. With something that small, no worry at all generally, but I'd just be disinfecting it, to hopefully avoid any 'seedy' bugs taking hold, and when it grows out to near the ground, it may break off in this area.
The 'fault line' ring about half inch or so down from the crack is *potentially* more of a concern, indicates a significant stress to the hooves at that point in time. Looks like there are many other far lighter/more minor 'stress rings' too. The slight flaring of the hooves also indicate things aren't perfect.
**These are minor looking signs, so nothing at all to panic about at all, but things to learn more about, so you can recognise problems, manage accordingly & hopefully avoid any significant problems.
Likewise the 'microcracks' down the grain of the hoof are not great, but not at all terrible either. Tend to indicate nutritional problems, or hooves that are in a wet environment. Or 'moisturised' with topical goops - hooves are meant to be dry.