they are horned worms and will devour your plants very quickly. All you can do is pick them off. I quit doing tomatoes because of them. I'll buy mine at roadside stands.
Hand picking them is the best option, as Kay says. You can also use Bt , or Bacillus thuringiensis, a live bacteria that is deadly to caterpillars and nothing else.
Use tweezers if you are squeemish put them in a plastic bag to squish.
We "dusted" our plants with "Bayer Advanced" Garden and Vegetable dust. So far (knock on wood) no bugs and our plants a healthy! Green worms(horn worms) are one of the pictures on the front of the bag!