General synonyms:
* Slippery
* Smooth
* Wriggling
* Squirming
* Sinuous
* Serpentine
* Elusive
* Gliding
Words with negative connotations:
* Sneaky
* Devious
* Cunning
* Shifty
* Dishonest
* Unreliable
Words with positive connotations:
* Graceful
* Fluid
* Elegant
* Agile
More specific synonyms:
* Wormlike (for creatures resembling worms)
* Snakelike (for creatures resembling snakes)
* Slimy (for a slimy texture)
The best word to use depends on the context and the specific meaning you want to express.