Pax,
I have read a little bit on this. It is a pasty buildup that will prevent the chick from pooping because the current poop sticks to their feathers and prevents the outflow of more poop. As mentioned, you should wipe it with a rag and water, but it can happen again. Another method is that you can just pull it off of the chick. It will probably take some of its down feathers with it and may be a little painful to the chick, but on the bright side, it won't build up again as the chick has no more down in its nether regions.
Hope this helps.
Ryan