This is called "pasty butt" (technical veterinary term) and is very common in young chicks.  What I found works the best is to have a little dish of comfortably warm water, place them in the water and hold them with one hand just so that is covers the poopy area, and gently clean it away with your fingers - it can be very dry and hard and will actually block the vent so that they can't poop, which can kill them.   So it's importatnt to do this.  They may resist a little at first, but mine quickly calmed and almost seemed to like it.  Don't try to pull it off, you can tear that fragile skin very easily.  Some people say to wipe it away with a damp cloth or paper towel but this seems to take forever.  Just be sure they have their heat lamp and are plenty warm after being in the water.  They'll get over this in a few days - it may be due to stress or dehydration from shipping - I've never seen it in hen-raised chicks, interestingly.