I had a crack like that on a horse I had years ago. I bought him from a low end auction. He had been locked in a stall for 2 years since his owners death and only sporadically cared for. He still had front shoes on his feet even though the toe had pulled them way forward. I literally cut the first 8 inches or so off with a hacksaw.

In hindsight I probably should have gone slower but I took a lot off in the first 3 or 4 trims. He was foundered when I got him. Knew enough to put weight on slow.
He proceeded to blow abscesses for the next year and a half. Sometimes they would cause large portions of the hoof wall fall away. He spent a lot of time in diapers and good duct tape. Even his rears blew a few small ones. The fronts had to pretty much create a whole new hoof from the coronary band on down.

Is that wet ooze from his hoof or some pasture/horse goo?

Hard to tell from those pictures but it looks like he needs a good trim.

That's the only time I've ever had a horizontal crack of any size. The others have been a small exit crack for a small abscess. Something to watch rather than worry. Those have been rare and far in between too.