Clear "worm" in horse's eye?

ok well i was out in the pasture with my 2 mares. i noticed the black mare had one eye that was running as if something was in it. i found a clear worm like thing onn her eye. it is invisible from more than 2 feet away. the mares are UTD on everything: shots, wormer, all that good stuff. could it be a worm? if so how to treat it? is the vet necessary? could it just be a long clear "eye booger" ? thanks!