Interesting...
I've seen tunicates, shrimp and and plankton all bio-luminesce(sp?) in the Gulf. It comes from the algae they are eating. Never heard that about Mantis.
How long since you introduced live rock or sand to your tank? It could very well be that this will subside over time as the food source is no longer present.
If not, I'd say you've got quite a money maker on your hands my friend. Everyone will be lining up to buy glow in the dark worms from you. In fact, I'd like to be first on that list!