Every now and again, composers who are employed at game companies may do work for other companies in their spare hours. Due to their non-compete clauses, they often have to use aliases/pseudonyms for these outside works. Here is a post cataloging cases of these that I'm aware of. This list does not cover aliases used by freelance composers for specific genres, such as eroges.
Namco:- Shinji Hosoe - used Yuji Yamada on several non-Namco gigs between 1991 and 1996, including the Little Master games and Eye of the Beholder.
- Takayuki Aihara - used Midori Maki on a couple of arrange albums in 1994.
- Satoru Kosaki - used Shinji Ikeda on both Gekkan Kin no Tamago? albums and the second MUNTO OVA. He left a few months later and continued to work with its director, Yutaka Yamamoto, on various projects at MONACA.
- Tetsuya Uchida - used tetsu-yeah for numerous song composing/arranging and DJ gigs between roughly 2006 to 2015 (he joined Namco in 2004, but used his real name for these gigs up until around the time NBGI was established), also used Tron-LM for the Love Live! song "Yuujou No Change".
- Rio Hamamoto - used oriori for several gigs for MONACA between 2010 and 2014.
Sega: (one case of this is quite significant in particular - hundreds of works under a handful of aliases spanning over a decade, but he is currently still employed at Sega and specifically wanted this info private, so to respect his privacy he will not be listed here)
- David Leytze - used Eva Delvis on 3D Shooting Tkool, a non-Sega game he worked on not long before he left Sega.
- Seiichi Hamada - used Daichi 'Ace' Hasemi for Tantei Jinguji Saburo games and Ato Titiyama for eroges. Both of these were used for over a decade, starting from around 2002.
- Hideki Naganuma - used skankfunk for a few freelance gigs between 2006 and 2007, before oddly enough - using it for Sega works during 2010-12, and by this point he had went freelance.
Misc:- Tommy Tallarico (Virgin) - well, this one is awkward. He "composed" (translation: got a network of ghost writers to do it for him) the music for Earthworm Jim, and instead of himself or even the ghost writers being credited, he asked for Mark Miller to be credited instead. While Mark did the SNES conversion, he had nothing to do with the Mega Drive version.
- Manabu Namiki (Raizing) - used Doukan Chiyoda for Shinshuku Taisen: It's a Noni!
- Atsushi Shirakawa (Falcom) - currently operates professionally as TENMON, but this alias was already in use for professional non-Falcom works he did before he left, such as Kanojo to Kanojo no Neko. Most but not all Falcom composers have operated under aliases both before and after leaving, such as Takahide Murayama (Eiichiro Yanagi) and Ryo Takeshita (Takepon Maki/Deadball-P). (Wataru Ishibashi used his real name at SuperSweep, but used mirawi for everything else outside of Falcom)
- Takahiro Ogata (Atlus) - may have used Takafumi Onoyama on Fullmetal Alchemist: Dream Carnival. Two tracks in the game are confirmed to be his work; one appears on an album credited to him, while another is based on a track from an older game he worked on. It appears he went freelance not long after the game came out, before quitting the game industry in early 2006.
- Michio Okamiya (AQ Interactive) - used Kimio Itoyama for Anata wo Yurusanai and Lord of Vermillion. This is weird though, as he worked at AQ as marketing staff and is credited as Okamiya in the former game.
- Tetsuya Shibata (Capcom) - used Toru Nakagawa for Half-Minute Hero, right before he left Capcom and founded Unique Note.
- Hidenori Iwasaki (Square Enix) - used Itsuki Iwasa for arranging a few Nobuo Uematsu soundtracks, such as Fantasy Life, as well as manipulation work on the KanColle anime.
There's probably many cases we will likely never know about, too. If there's any that I haven't listed here that you're aware of, please let me know. Also interested in speculated matches too!