I am not an expert on this but I would monitor the toad carefully. Check for behavior/eating changes and signs of discomfort. It is hard to see from the pic but it could be from a number of things. The toad may have something lodged in its eye, it could have a corneal tear, bacterial infection, or it may be the protective eyelid that comes over them when they sleep etc. It looks like a cataract but I dont know if that is even possible. I personally would quarantine the toad to make sure it is not infectious and to keep the area sterile and schedule an appt with the vet if it has not resolved in a few days.