Horses like every other animal have tear ducts and when these ducts get dirt or something in them they'll clean themselves out. The goop is just a result of the horses eyes trying to keep them clean. I wouldn't worry about it. =)
Mine all take turns having some eye goop in winter due to hay feeder/dust-dirt, I believe. As long as there isn't signs of tenderness or pain it isn't an alarm bell. Squinting or excessive runny eyes would be tho.