The worms are transparent their entire life! 🙂
This is because they are so small and not made up of many cells (only about 1000 in an adult worm) and none of the cells contain any coloured pigment (no coloured stuff), so they are transparen.
The worms that glow green are ones that are made in a lab by putting the GFP (green fluorescent protein) gene from jellyfish in them.
In theory the worms could be born opaque and become transparent later as they go through molts at each of the larval stages as they grow. This is when they shed and renew their cuticle. But to do this the constituents of the cuticle would need to change.