Does This Brown Patch on a Boy’s Buttock Signal Underlying Illness?

Case:

A 12-year-old boy has had this brown patch on his left buttock since birth. His mother is concerned that the patch has grown over the years, and she wants to be sure that it will not be a health concern for her son. The mother has not noticed lesions elsewhere on the child. There is no family history of skin disease.

Does this patch indicate an underlying illness? Should it be excised to avoid cancerous change?