An update. I went out there today and I swear the crack was worse, which is bad. On the other hand, the farrier was there, which is good. The barn owner was like, "Oh, I have seven horses that need shoes today so I'm not sure he can fit Teddy in." I was like, "too bad." Seriously, I have never put made a stink about anything, but the crack needed to be looked at.
The farrier agreed that the wet-dry cycle of their pasture is probably contributing to the crack, but I told him I can't move him from that pasture and don't want to stall him, so he recommended either Durasole or White Lightning (he said if I mixed the White Lightning with vinegar and put it in a soaking boot, it would make vapors that would really get into the crack and clean out any germs that might be there). I have ordered both. Any thoughts about which is better? He also bevelled the hoof around the crack, and recommended that I get a tiny pick to make sure I keep the white line area clean. He didn't see anything to worry about at this point, but felt like some preventative measures might be in order.