pseudonymous

pseudonymous \soo-DON-uh-muhs\, adjective:

1. Bearing a false or fictitious name.
2. Writing or written under a fictitious name.

I did a lot of pseudonymous writing during this period.
-- Henry Miller, Tropic of Cancer

Unless Ollivier was playing a doubly pseudonymous game, the cipher was no good.
-- Paul La Farge, Haussmann, or the Distinction

Pseudonymous actually predates the more common word pseudonym which was coined in the early 1800s. The longer adjectival form of the word arose in the 1700s directly from the Latin word pseudonymus meaning "falsely named."