It appears that there is corneal edema which is improving with the drops. The corneal opacity could represent a couple of possibilities. First might be corneal scarring secondary to inflammation. Another possibility might be a sequestrum. While sequestra is typically dark in color, some can be lighter. While it is a very good picture, eye problems can be difficult to diagnose with a picture (or in person too!). Here is a link to sequestra information - you can read it and see if it may apply (it states breed predilection however any cat can acquire them)... http://www.animal-eye-specialists.com/sequestrum.htm
I do like your veterinarian's idea of an ophthalmologist referral. Some of these corneal opacities can benefit or require surgical debridement. I hope this helps.