Does the Alarm spell trigger if creature is on the ethereal plane?
The Alarm spell doesn't have a clause to say otherwise, but other magic doesn't cause damage effects on the ethereal plane unless stated in the spell.
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No, Alarm can't detect ethereal creatures.
While it is not explicitly stated, each of the planes is separate from each other, including the Ethereal and Material planes. From the 5e srd:
The Ethereal Plane [...] overlap[s] the Material Plane and the Inner Planes, so that every location on those planes has a corresponding location on the Ethereal Plane. Certain creatures can see into the Border Ethereal, and the see invisibility and true seeing spell grant that ability. Some magical effects also extend from the Material Plane into the Border Ethereal, particularly effects that use force energy such as forcecage and wall of force.
So we have two planes that are completely disconnected from each other, except that sometimes you can see from one to the other, and certain effects reach from the Material to the Ethereal.
Since only some magical effects reach through the planes, we must assume that every spell does not except for those that specifically call out that they do (like force cage).
Since Alarm doesn't state that it detects creatures on the ethereal plane, it doesn't.
Aside: In reference to the original scenario, one could transfer to the ethereal plane, place the Alarm spell, and wait there if one wanted to catch a Hag.