Does she have a water dish? (I am assuming yes...)
I would take a guess that she either decieded to play in the water or fell into the edge of the water bowl and it dribbled down her chest.... The dander and what not in the brooder would help make it crusty too. Ususally a chicken will just preen and use their oil to get the feathers back to normal but perhaps this chick hasnt figured that out yet?
I have a 8 month pullet (a silkie, which yours appears to be also) who likes to sit in the open water dish (about 2 feet across and no more then 2 inches deep) for semi-long periods of time. She has that plastered down/crusty look to her feathers sometimes when she decieded to forgo preening afterwards and just goes running out of the water and gets busy with something else. My girl just has a few loose screws in her head, but from what I hear, that is a common silkie thing too.
Basicly, unless she is acting sick in any other way, I wouldnt worry about it yet.