What you describe can be an entirely normal process called a halo nevus.
A halo nevus is a lesion with a centrally placed nevus (mole) surrounded by a white halo of depigmented (loss of pigment) skin. Over a period of months to years, the halo nevus gradually becomes smaller and disappears, often leaving a residual white patch.
This relatively common disorder occurs between the first and fifth decades of life, most frequently during late adolescence. Halo nevi can occur almost anywhere on the body, but most often appear on the trunk, especially the back. As many as 50% of affected individuals have more than one halo nevus.
The phenomenon seems to follow several stages. The first is a sudden appearance of the white halo around the mole. In the following months or years the central nevus disappears. Finally, the white patch may re-pigment spontaneously and return to normal skin color. However, the white area often remains.
Halo nevi are quite distinct in their appearance. Most halo nevi are seen with benign (noncancerous) moles. However, in rare cases, halo nevi are seen around a malignant melanoma. It is therefore important for a dermatologist or Dr. Johnston to examine the central mole. ip info If it appears typical, it can be tracked and evaluated at regular intervals. If the central lesion appears atypical and suspicious, it should be removed and biopsied for microscopic evaluation.
Halo nevi are considered harmless and self-limiting and are not treated. If the central lesion is problematic in appearance, however, removal of the mole with its surrounding halo is recommended.