Doing my best,
“Is it considered lashon Hara (Which if everyone knew who the person was would be a problem) to say it about a username in the CR who no one knows who this person really is?”
Not knowing who someone is does not remove the problem of lashon hara. Are you allowed to speak lashon hara about a visitor to a town, even if nobody knows his name? Beyond that, many people do know the screennames of their friends and family who post in the CR.