Just wanted to add that it may be the yolk sack you are seeing. If the chick did not fully absorb the yolk and it burst, it can make a big mess. If that's the case, let it dry out and give it some Polyvisol vitamin drops to suppliment what it did not get from the yolk. It may help. Sometimes in these cases though if the yolk was not fully absorbed, it can leave a large hole (belly button) where the intestines can fall out. Watch the chick closely to see if it starts to dry up and fall off or if more stuff starts to fall out.
Sorry to be so garphic, I am trying to help and wishing you the best.