Croton may be diseased
This Croton appears to have a aucuba ringspot virus. Is this your opinion also? If so is there an effective treatment?
Your plant actually looks quite healthy and it seems to be losing it's color variations due to low light conditions.
The bright spots your seeing on the leaves are just the areas that still have the color. I see no disease and the only treatment needed is increasing the sunlight that your plant receives.
These links have more information.