Looks like poop that has got stuck to the chick fluff to me. Cleaning it up should stop any more being added to it. Looks like quite a glob so you may have to soak it. I've found a wet warm water cloth pressed onto the poop helps dissolve it.
Good luck with your silkie.
"Happiness can be found, even in the darkest of times, if only one remembers to turn on the light."