non-de-plume
a pen name (nom-de-plume) that people think is yours but actually isn't.
"Have you seen that Bob Jones guy's new book? I think that it's actually written by Fred McDougal."
"No bro, that's not his book. You're assuming a non-de-plume"
"No bro, that's not his book. You're assuming a non-de-plume"