That is called pasty butt and is sometimes brought on by stressful conditions-shipping would be one! I used to take the chick and rinse its vent under a trickle of warm running water. Then after drying it, either blotting with a towel or using a hairdryer or both, put it back in the warm brooder. It may take a couple of times but it will eventually go away, you will probably pull some down off its butt, but that would be normal. You can try putting a teaspoon of organic apple cider vinegar in a quart of water, this will help their system to balance out. Good luck!!
You must clean it off, it will not be able to poop with their vent blocked and it may cause its death.