What is 'Musk' as used in this recipe
It's nutmeg. 

The author of that blog is from Switzerland, so I imagine that term is used there, but I had never heard used culinarily until now. I Googled "Grated Musk", and still had to look around to be sure. Thanks for teaching me something.

EDIT As of an hour after the question was posted: Click the "Grated Musk" link now! This question is now the top result :) You're famous, Tom!