Pollen --- his nose was FULL of yellow pollen. He had clear liquid running from his nose and it was catching that yellow pollen on its way out - yuk yuk and more yuk.
I have four horses and a check of everyone's nostrils revealed pollen in the nostrils and it had gotten way up into their nose cavities -- even the horse with the grazing muzzle wasn't exempt
So a bunch of Vetericyn-Sprayed paper towels later, everybody had the pollen boogers completely removed. A water damp paper towel with a bit of Witch Hazel added, will also work.
We live in the Tennessee Valley and the pollen is so thick right now, it looks like fog at night, when we shine a flashlight. The water tubs have a layer of pollen on them every day and my car is not covered in dust, it's covered in pollen.
The point to this is, while most don't live in an area where the pollen just hangs around for days until a tornado wind comes along, or it just reaches its end of life, check your horses noses for stuff that doesn't belong in there. It could be why the horse is itching its nose more than usual
And don't forget the ticks - I was pulling those out from under the jowels too
All this and spring grass founder worries -- I'm having a hard time remembering why I used to look forward to Spring