star trek - What is this trail of 'pixie dust' left when the Enterprise warps out?

This could well be plasma residue which is a byproduct of warp drive engines.

In previous incarnations of the star ships from the films and TV shows, it was revealed that the plasma residue needed to be purged from the ships' warp nacelles on a regular basis to avoid dangerous build-ups of the substance in the warp engines.

As the plasma residue has always been portrayed as blue matter, the same color as the 'pixie dust' seen in the latest film, I think it is safe to assume that the plasma residue is being purged simultaneously with the activation of the warp drive - resulting in those lovely trails.

Information disseminated from the official Star Trek wiki - here and here.