Nutritional deficiency maybe a possbility here. You can try to vary the diet further by adding squid,fishmeat(in addition to the silversides, though fishmeat can get messy) to the diet. You can also look at enriching the food with additives like Selcon.
What is the tank lighting? I've read before it can cause cloudiness in lionfish eyes.
If the fish is still eating, I would suggest maintaining or improving the water quality and feed a more varied diet as a first step.
I dont really think water quality is the cause here, the nitrates shouldn't be too high for this fish.
I've read of eye flukes before, it causes such cloudiness but I'm not certain if this could happen to saltwater fish. I only know it can happen to freshwater fish. Maybe introduced through the live molly diet? Again, I'm not sure if this disease exists for saltwater fish.
Maybe you can checkout this thread on WWM as well:
http://www.wetwebmedia.com/dwfliondisfaqs.htm