If it is sitting on the surface, then it is most likely an ulcer that has resulted from an injury; it could also be a deposit of fat or calcium, but that is for a vet to decide. If it is inside the eye, then you are dealing with a cataract, for which there is no treatment. Please see your vet for a proper diagnosis. Potential eye injuries should never be taken lightly.
PS: As we are delaing with members from all over the world, you can help us a lot to give you advice that is tailored to your location straight away (even more so in an emergency) by adding your country, state or (for the UK) your county/city to your details. Please click on your username on the top bar, then go to personal details and scroll down to location. Thanks!
