As far as I am aware worms are good creatures and do no harm. i really wouldnt worry. There probably just keeping warm.

If you are referring to caterpillars as "worms", there ar a few species that will harm tomatoes, and it depends on if the mother moth finds her way into the greenhouse--easy for her when the greenhouse is vented. Check often for chew marks, and the little black-green pellets of their droppings, so you can deal with them while they are young.