What are you seeing that you think is a worm? I'm not seeing anything obvious, usually only roundworms or tapeworms are easily visible, I'm not seeing either. I see undigested grains/seeds and some grass or greens. Can you get a fecal float test done? That is the best way to confirm a parasite problem, most of the time they are not visible and evidence is microscopic. Are they acting OK other than the dirty butts? Lots of things can cause dirty behinds or runny droppings, parasites are only one.