The cloudiness, if it's the cornea, is from inflammation and that can come from scratches, abrasions, infection, ulceration, etc. Do not put anything other than sterile saline solution made for eyes in the eye until your vet comes and examines the eye because if there is any damage to the cornea you can make it worse by putting the wrong sorts of things in.
Eyes have poor circulation which means that they don't heal as well or fight off infections as well---remember, the blood is what brings in all the things needed to feed cells and carries the white blood cells and antibodies that fight infections. So, getting the proper treatment as soon as possible for an eye is really important. A scratch or ulceration on an eye can lead to a severe infection and perferation of the eye in a matter of a couple of days.
Keep your horse out of the light, use a fly mask to help provide protection for the eye and get your vet out as soon as possible.
Licensed Veterinary Technician