Abbreviation of 'alternate account'—internet slang for a secondary social media account. Informal and casual.
別アカ持ってる？こっちでは本音を話してるんだよね。
Do you have a spare account? I say my true feelings there.
最近別アカで配信を始めた人が増えた。
Recently more people have started streaming from alternate accounts.
誤爆したくないから、重要なやり取りは別アカでしている。
Because I don't want to post to the wrong audience, I handle important communications from a separate account.