The question is regarding the succubus' Etherealness action:
Etherealness. The fiend magically enters the Ethereal Plane from the Material Plane, or vice versa. MM 285
While the succubus is in the Ethereal Plane, can you see her?
Does her Charm ability work in the Material Plane while she is in the Ethereal Plane?
Can she travel to the Deep Ethereal?
The description below Beautiful Corrupters (MM, p. 284) and the description of the Ethereal Plane on the DMG (p. 48) seem to contradict each other.