Sounds like normal mucus discharge (the "lubricant" to get the poop out).
If he is eating, pooping normal poop and so forth.. I would just make sure he has adequate water intake since he is likely increasing his hay intake due to it being winter.If there is snow where you are, don't expect him to eat snow for his water intake (you probably are not doing this but you would be surprised the number of people who DO think a horse can eat snow in winter and does not need water so I lay that out there more for other readers!).
Take his temperature, take his pulse (you can feel it under his jaw) and watch his resting respirations. If all that is normal, then this is also likely normal.
There is something about the outside of a horse that is good for the inside of a man. ~Winston Churchill
) Dinosaur Horse Trainer