Tyke had these all his life in both eyes - different vets told us different things, the ones we trusted (those who were interested in animal care, not the amount of money that could get out of loving owners!) were not worried as they did not seem to bother him in any way. They never spread for the whole of his almost 12 years and never seemed to affect his sight.
Now Barney seems to have the same thing, but only in one eye. Again, it doesn't seem to affect him and the vet has not commented on it when he has his check up.
I was just starting to wonder if it was another Boxer thing......along with those plugs in their ears that stop them from hearing you when they want (or don't want) to do (or not do) what you're telling them!
It's wise to get it checked out, as it could be something different, but in our experience so long as it isn't spreading or affecting them, we've learnt not to worry.....hope that helps ease your mind ;-D