are they pot worms? do a google on potworms and see if that is what you have.

If so they are harmless and are simply there because the mulch is decaying and they eat bacteria in the decomposing material just like other worms do.

They are actually good to have around the same as any worm is.

Pot worms are usually around half inch to maybe an inch long and thin.

If what you are seeing is consistently thicker and shorter then perhaps some sort of maggot? Was any manure or meat product incorporated into the soil? If so I would suspect that.

Well, that's it for my guesses. ;-)