Serapheem

The  were a benign race of microscopic, extremely light creatures that had the power to deconstruct the molecules of other species. They were briefly encountered by the Ninth Doctor in 2004.

Biology
The Serapheem could not be seen by the naked eye and were lighter than Earth's air, allowing them to undergo their intergalactic migration on starlight. Every ten years, a group of Serapheem left the Helios System to migrate through the Zenthantine star system, during which they stopped by Earth, but were harmless enough to normally never be noticed.

They were a helpful species but could not distinguish between speech and thoughts, causing them to try and fix humans' inner problems by deconstructing their molecules and taking them with them on their migration. However, humans were still unable to fly on starlight in a deconstructed state, causing the Serapheem to leave them behind after realising they could help no more. Although they were unable to return humans to their normal state on their own, they could broadcast the instructions for other people to do so, though the time limit to do this was only around 24 hours.

The Serapheem had a swarm consciousness and therefore could not be natively translated by a TARDIS without outside help.

History
After spending several decades going on migrations across the universe, the Serapheem were eventually forced to permanently leave their home six years early due to the expansion of their star. They sent out an SOS signal which was picked up by the Ninth Doctor, causing him to land in 2004 Dundee to investigate.

While there, they deconstructed several dozen teenagers who wished to escape each time they visited, which were treated as Missing Persons cases by the police. The Doctor eventually used a pulse resonance chamber to allow the deconstructed Marnie McDonald to talk to the Serapheem, allowing them to realise what they were doing was wrong and broadcast the set of reconstruction instructions for each person. The Doctor used this to recover the teenagers who were recently taken and the Serapheem moved on. (AUDIO: Girl, Deconstructed)