Nevus simplex, commonly known as a "salmon patch," is a type of birthmark that appears as a flat, pinkish mark on the skin. These marks are often found on the back of the neck, eyelids, or between the eyes. They are caused by dilated blood vessels and are usually harmless, fading over time. These birthmarks are most common in newborns and are more prevalent in those with lighter skin. Nevus simplex does not require treatment, as it typically does not cause any health issues. In most cases, the marks become less noticeable as the child grows.