The speaker insists, almost too firmly, that the ghosts in this poem are not sinister. They sound gentle: pale hair, smiling lips, feet that murmur in the grass, songs drifting across the twilight. But the details keep undercutting the reassurance. Their lips are "bloodless," their bones are hollow, and the wind blows straight through them. They are paler than rain, barely there at all.
The real shock comes at the end. The travellers who stop to listen to the singing have been turned to "old stones" scattered across the islands. The charm is a trap, and the calm, matter-of-fact tone makes it more chilling. The repeated "Oh no!" sounds like someone protesting a little too much, or perhaps mocking the idea that anyone could be afraid.
The mood is eerie and dreamlike, soft on the surface and cold underneath. The poem suggests that the most dangerous things can be the ones that seem beautiful and harmless.