New York City police are searching for a missing 76-year-old mother who got separated from her husband on the subway late Tuesday. Octavila Lopez, 76, was last seen around 5:30 p.m. riding a southbound 5 train at the Newkirk Avenue-Little Haiti in Brooklyn — about a mile from her home in East Flatbush, police said. Lopez, who suffers from Alzheimer’s disease, is described by police as 5 feet 6 inches tall and 140 pounds with brown eyes and gray hair. Lopez was last seen wearing a black jacket,