One of them must've at some point, it's genetically (nearly) impossible for this chick to come from two Silkie parents. Bear in mind hens can sometimes be able to store a roo's sperm in her body for up to months at a time, so any contact she could've had with another rooster in the past couple months would've produced this chick. Does it have other silkie features? I see it has feathered feet, does it have the extra toes? The head floof looks a little lacking like it only has one copy of the gene.
What other roosters do you have? Like what breed. Or do you neighbors have any? As the chick grows you'll be more able to definitively tell who/what the father is.