I can't remember if you posted their history before or not, but the only thing that comes to mind (since there are no current mutations for such a thing) is that it could possibly be a new mutant. One that strikes me as similar is the "pink eye" gene in mice, which causes them to have red/pink eyes, but also really affects their ground coloration by lightening it.
Could it be at all possible that your ghosts may be the first known with such a gene in corn snakes?
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"If more of us valued food and cheer and song above hoarded gold, it would be a merrier world."
J.R.R. Tolkien

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