Is that from the pair behind the baseball field? I was there in February and noticed they had those "marble" eyes instead of the yellow ones. Don't know the reason, but looks cool IMO.
I don't know what is causing this effect in the eye, but here is another burrowing owl image that I posted in the Avian forum about a year ago. If anyone has an explanation, I too would like to hear it. (This was an adult bird paired with a normal-looking owl.)
When I have human patients with this effect we call them iris freckles. It is just more pigment showing through in spots. Pretty much normal in humans - I don't know about owls.
Athene cunicularia floridana is the subspecies of Burrowing Owls found in Florida and the Bahamas... the best I can find states "Yellow eyes (irises) are most common; but chocolate or olive irises have been documented." So I am going to assume this is just a rare trait that occurs in this subspecies, maybe just a different 'morph.'