My old boxer's eye had discharge, now it's cloudy. Is it serious?

Oh dear! This sounds like something that definitely needs to be seen by a vet as soon as possible. Discharge from the eyes can be caused by an eye infection, an injury to the eye such as a corneal scratch or ulcer, allergies, or even chronic problem such as dry eye. The fact that one eye now looks cloudy is very concerning - this could be due to a severe corneal ulcer, an infection or inflammatory reaction inside the eye, glaucoma, or a cataract. Eye problems like this need prompt attention because there is a risk of permanent damage or even vision loss the longer it goes without treatment. Because of this, I would recommend taking Diesel to an emergency vet today - he/she can examine his eye and use a special stain if needed to check for an ulcer or other injury, and will prescribe appropriate medication to treat the problem depending on what is found.