Guessing silver base colored bird with mottling. Not sure what other pattern genetics are going on with that one.
That was my thought, too. To my eye, the cockerel (Casper) is like the pullet (Cowabunga - my daughter named them, what can I say?), just more white and less color. Their parents were siblings, so while there may be a random mutation in the breeding stock, there's also an excellent chance of getting some recessive trait expressed, or a codominant one to really get expressed.
I know that donkey/mule genetics can cause horse spotting patterns (like Appaloosa) to express in eye-popping loud ways on mules (they call it "skewing"); I'm wondering if Seramas may have modifiers that skew the expression of otherwise well-known genes. DD tells me she has heard other Serama breeders refer to "dalmatian" birds; maybe this is like paint in Silkies?

@bullets It will be interesting to see what kind of results you get. When I have seen people talk about setting up breeding pens, 3-4 weeks is the longest I've seen anyone think was necessary to wait to be sure that the eggs being laid were fertilized by the male in the pen.