how old is the chicken in the pic?
do you have young ones and older ones together?
looks like a bad infection..most likely from pecking..looks like a peck mark/gash under the eye
please try and separate the injured ones and any younger ones away from bigger ones...they will continue to be bullied and they are vulnerable..
can they even see to eat and drink?
what all do you feed?
also possible they have a bad case of conjuntivitis..but still vulnerable and need protection, and easy access to feed and water..
try and get some vitamins for the drinking water..
what bedding do you use?
describe their droppings..in detail..color and consistency.
is the yellow "muck" like a hard crust, or is it soft and mucusy?
do they have mucus coming from the beak nostrils?
have they been treated for mites/lice?
get some saline solution eye wash..not expensive at any pharmacy..plain saline eye wash...not for "getting red out"..or any other additives..ask the pharmacist if you need help..
get some terramycin eye ointment at most farm/feed/ livestock stores.
gently squirt out the eyes using an eye dropper with the saline eye wash, and try to clean off the "muck" with a soft piece of gauze..not cotton ball...continue cleaning out all muck and debris.
dab to dry around the eyes.
apply ointment as directed on package..
here is an online source for other eye ointments::
http://www.firststatevetsupply.com/store/index.php?cPath=21_57