I have never really heard of a horse getting a bacterial conjunctivitis, but if you were a human patient coming into my office with green goop coming out of your eye, that would be my first bet.
And the fact that it has spread from horse to horse supports the fact that it may be a bacterial infection.
Traditional "pink eye" is a viral infection and usually does not have green goop. Pink eye usually has a lot of watering and sensitive to light with a burning sensation (obviously our equine partners can't tell us if their eyes are burning).
You can try to let it run its course but if its already been a week and it hasn't gotten better, it may be wise to put a anti-bacterial ointment into the horse's eyes to try to kill off the bacterial infection. See if your vet will prescribe you something you can use for your horses.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.