Does Malevolence need an ethereal ghost that is manifested as opposed to a non-ethereal ghost? Is it possible to un-ethereal a ghost? I thought it's always ethereal, and incorporeal too when manifested only.
Is this a case of bad wording or errata or type-error?
Once per round, an ethereal ghost can merge its body with a creature on the Material Plane. This ability is similar to a magic jar spell (caster level 10th or the ghost’s Hit Dice, whichever is higher), except that it does not require a receptacle. To use this ability, the ghost must be manifested and it must try move into the target’s space;
A ghost dwells on the Ethereal Plane and, as an ethereal creature, it cannot affect or be affected by anything in the material world.
A ghost has two home planes, the Material Plane and the Ethereal Plane.
So ghosts are Ethereal simply by virtue of living on the Ethereal Plane. A ghost could plane shift or gate from the Ethereal to the Material Plane and stop being Ethereal (or for that matter incorporeal, since it can no longer manifest - a ghost may only manifest from the Ethereal Plane, and is not incorporeal unless they manifest).