We have a hibiscus in a pot on the window sill and masses of powdery soil is
regularly appearing in the saucer coming out from the bottom of the pot.

We are assuming it's a worm? Is it detrimental to the plant in any way and
should it be fished out? One curious aspect is that its pushing out so many
of the little soil granules that the pot no longer even sits upright....
that's a lot of serious pushing out.