It seems that you may be having nevus pigmentosus.
It is a light brown to dark black patch. There may be small melanocytic nevus
or mole in the patch.
It is a benign condition. It is only concerned cosmetically.
So there is no treatment needed.
If it is embarassing, you may go for aesthetic treatment... Like laser therapy
Otherwise , you may live with it.
I hope you may be satisfied with my answer.
Dr. Ilyas Patel MD