A white patch on the arm with a black spot in the center may be vitiligo, or it may be a halo nevus. 1. Vitiligo: Vitiligo is a primary skin pigmentation disorder, mainly due to the destruction of melanocytes in the body, in the progression of vitiligo pigment loss is relatively fast, the local pigment residue may occur, resulting in black dots in the middle of the white spots. 2. halo nevus: halo nevus is a limited hypopigmentation around a pigmented nevus, mostly in the trunk of the body, the main manifestation of the nevus is surrounded by a circle of hypopigmented spots, varying in size and white color. There may be other reasons for a white spot with a black spot in the middle of the arm, you can go to the hospital for Wood’s lamp, skin confocal microscope and other examinations to identify, and then according to the results of the examination to carry out the corresponding treatment.