I have figured out what the rust-colored stuff on Gabe's face was. He's come in with it a few more times and I've noticed the stuff on my boots, too. Even his tongue was a bit orange-colored last night.
It's grass rust. Fun, eh? I found some of the spores on the fescue and the perennial ryegrass, but not on the timothy, orchard, clover or alfalfa. I've searched the web for info about this stuff and all I've found is info for people who have it on their lawns, not in their pastures. Is it toxic to the horses? I have no clue. I haven't noticed anything off about any of them, but I'm still a bit worried about whether it could be toxic to them.
Until I can figure that part out, I've pulled them off the most-affected pasture for now. I hate doing that because otherwise, my pastures are all still growing and gorgeous. I hate losing all that grazing time! Less grazing time means I have to feed more hay, ugh. I hate having to feed more hay before I actually have to feed more hay because the grass is gone.