Viacom Inc.'s Nickelodeon unit said Monday its president, Cyma Zarghami, will step down after 12 years in the role, and 33 years at the company. Sarah Levy, chief operating officer of Viacom Media Networks, will lead Nickelodeon until a permanent successor for Zarghami is found. Zarghami had joined Nickelodeon in 1985, and was named president in 2006. The Nickelodeon network is now in its 39th year. Viacom's stock was up 0.6% in afternoon trade. It has slumped 12.8% year to date, while the S&P 500 has gained 2.7%.