My 3 yr old rottweiler has a purple cloudy spot in the corner of his left eye. What is it?

More than likely it is a Cataract forming.

Vets can remove these now.

I agree with Donf, chances are it's a cateract. Could be a stye, but best to have it cheked out by the vet anyway.

Is the spot on the outer clear part of the eye or inside the eye? He could have an ulcer on the cornea. Take him to the vet and get it checked out. Don't mess with eyes!