Yes, remove it, it's a sport or reversion and likely more vigorous than the kojo no mai, if left it will likely become dominant and may well take resources from the rest of the plant, potentially weakening it. It's the same approach you'd take if you had a green leaved branch grow on a variegated plant.
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It will need removed if it is.
I live in west central Scotland - not where that photo is...
"Have nothing in your garden that you don't know to be useful, or believe to be beautiful."