Can a Magic Circle Against Evil block the night hag's Dream Haunting (Su)? The dream haunting has the flavor of "exercising mental control over the creature," but the dream haunting description only seems to rely upon the night hag hovering over the creature.
No, it will not protect the recipient of the Magic Circle against Evil.
The Magic Circle only does three things, none of which impede the Night Hag:
Prevents summoned creatures from entering the area. The Night Hag isn't a summoned creature.
Prevents domination-type mental control over the target's actions. The Dream Haunting doesn't exert any kind of control, let alone metal control of physical actions.
Grants a bonus to AC and saving throws against attacks or effects from any evil creature. Unfortunately, Dream Haunting doesn't grant a saving throw so the bonus doesn't come into play.
So the Night Hag can ethereally enter the Circle just fine, can hang about and trouble the victim's dreams so that they lose CON in the morning, and can do that without granting a saving throw. If the victim somehow did get a saving throw (perhaps from a magic item that gave saving throws against things like this regardless of whether they normally get one), only then would the Circle have any effect, and then only a +2 to the save.
No, because that is not ongoing mental control. Protection from evil states that
the barrier blocks any attempt to possess the warded creature (by a magic jar attack, for example) or to exercise mental control over the creature
It does not ward against any mental attack, just those that result in mental control. The dream ability is simply an attack against the creature through their dreams; it doesn't let the hag tell the creature what to do, and therefore protection from evil will not help you here.