Well then fly mask and wipe eyes when they go into and out of stables. If swollen and thick mucous/pus then call the vet. You could lose an eye if left to its own devices. (Happened to me decades ago).
Yes flys, yes dust, it's just clear
Depends. Do you have flies? Are they runny with a fly mask? Is there a lot of dust? Is it more puss like or just clear fluid? Could be easy to remedy or something worst. Essentially if it's something I am not familiar I call the vet.