secondary account; alt account; backup account
Internet slang abbreviation of サブアカウント. Used for social media, gaming, or any online service where a user maintains an extra account separate from their main one.
See also: サブアカウント
I post my true feelings on my alt account.
I made a secondary account for the game and started over from the beginning.
Abbreviation of サブアカウント (sub-account), with 垢 used as ateji for アカ. The kanji 垢 normally means 'dirt' or 'grime' but is repurposed here purely for its reading.