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).
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.