How does a decaying eye react to sunlight?

Photophobia. If you mean an eye that has an illness or disease, it can often cause the eye to be sensitive to sunligt. In an older person, progressive increase in sensitivity to light can mean a cataract or other eye disease is present.
"Decaying"? Not sure what you mean! If an eye is inflamed or infected, sunlight will cause discomfort and tearing. This light sensitivity is called photophobia.