It can be a disease, but more often than not the chicks just miss and poo on themselves. I just rip it all off like a bandaid. It plucks the feathers though too, but that makes re-occurrence less likely. If you want to be nice, get a paper towel moist with warm water and gently rub it off.
Yep - I stick their little hiney under the warm tap water running and let it soften and pick off the pasties with my fingernail. Pretty soon it won't be chicken poop - it's just part of your normal day
I haven't been raising chickens long but I would say yes wet their bottoms. When mine were babies I had to do it once with a wet/warm paper towel. Keep doing it until it softens. Be gentle because i've read the vent area is very sensitive. Make sure you dry them up real well so they don't get a chill. Hope this helps. Good Luck
Also, when you solve that problem, make sure that their poop isn't watery. If it is they might need a change in diet or might need some medicine or something. I'm sure someone with more experience will see this and get back to you soon.