Ok I am answer the best I can(since work blocked tr).

1)ur dog has glasses on, nething with glass, or a lens will get a bounce back and create that.

2)now the halo would be seen with out a flash. With the flash being shot at or near the eyes will create that strange color due to the pigment in the eyes.

3)most halos in dogs eyes usually mean a deaf dog. But it is rare.

--i am not a vet, wife works with animal control and knows a good amount about animals.--
So u can take my word or take it to the vet.

But a tumor I doubt it. Most animals will act very different with a tumor.