Question
I have had for many years maintained a home greenhouse in which I grow primarily epiphytic and miniature cymbium orchids.  The orchids are potted in fir bark and lava rock.  But the worms generally don't appear until the fir bark breaks down.
I also grow a few other plants - including, among others, amaryllis, agapanthus, Meyer lemon, and ferns. The worms frequently appear in the dirt in the pots of those plants.
I maintain a fairly damp greenhouse - it mists automatically for about 10 minutes 2 times a day.
I am writing about small (about 3/4 inch) reddish brown worms in the pot.  Sometimes they are under a pot when I lift it up.  The worms are slightly shiny, have lots of legs, and what appear to be antennae.  They have clear segments. When I pick one up with tweezers and put it on another surface, it curls up into a small tight ring.
When I pull up the moss that covers the surface of a pot, I will see the worms.  Then, when exposed to the light, they quickly dive into the dirt.

Answer
Joanne,

I think your are describing a small centipede. Centipedes this size are harmless (larger ones can bite). They are predators and are probably feeding on small insects in the greenhouse. They won't damage your plants. Centipedes come in a variety of shapes and sizes (google "centipede" or go to www.bugguide.net).

I hope this is useful.

Jack DeAngelis, PhD
Extension Entomologist (ret.)
author Living with Bugs: http://www.livingwithbus.com/bug_book.html