I don't know much about this plant, but I would remive that yellow leaf. If you keep a dead leaf on, the plant is still giving water to and spending energy on that leaf, trying to make it grow, even if it's dead.
Good lighting is a must for the variegation. My Birkin sits just at the edge of where the sun stops at a south window. In summer it's to the porch which faces north. She is so happy.