Remove the entire branch, not just the leaves. This is a reversion to the ordinary unvariegated form of cornus controversa. If it's coming from very low on the tree - below a swelling on the trunk - , the variegated cultivar was very likely grafted onto ordinary c. controversa root stock which is putting out the branch. Reversion can also occur higher up and should be removed as soon as it appears.

Thanks akamainegrower - that's good to know!

I'll remove the entire branch.