Some chickens just tend to have more messy rears than others. Dried poop on their feathers is not a primary symptom of worms. If it stays there too long, they can lose some of their feathers under their vent due to fecal scald.....dried poop. If it gets too bad, I put the chicken in a bucket of warm water and gently pull the stuff off of them.
While I would not say it is a symptom of worms, it often IS a symptom of lice. Lice eggs are laid to the feathers under the vent, are somewhat sticky, and poop adheres to them, then morelice eggs, more poop and pretty soon it is a huge mess.
Wash off, and if you find anything gritty looking or feeling, it is almost certainly lice eggs, meaning you need to treat the bird and all its coop-mates for lice.