Iguanas are susceptible to a bacterial infection in the epidermis that will cause a hardening, discoloration and even peeling of the skin. You can see the result of this in many older pet iguanas, it is the most common cause of the white or yellowish scars and patches frequently seen.
If this happens in the toes or tail it can result in the loss of that appendage. Uncontrolled it can cause death.
It is impossible to tell what the problem may be in your pet just from the brief description you gave and without any knowledge of how it is kept or the history of the complaint. But if the spots do not go away and appear to get larger or are associated with swelling, peeling or hardening of the area take the lizard to your vet.