Does it look like a milky spot? Or does it make the pupil look blurred?
In that case it could be either a nutritional problem, or a sign of bad water quality or even infection, so if I was you, I'd test the water's pH, ammonia, nitrite and nitrate, and perform a large water change of 50-70%! Please post numerical results of the tests as well!
Could be that they're not used to the "new" water when they're used to the one in the pond