Malicious accounts are accounts that Arctic finds potentially malicious by looking at some of their user information. By looking at the data, Arctic can generate a score, and if the score is above a certain threshold, Arctic will say the account seems malicious.
Yes, you can. Use
a?userinfo <user> and look for the Malicious Account field.
Yes, you can make Arctic mute, kick or ban people with malicious accounts as soon as they join the server. They'll receive a direct message notifying them about the action, and Arctic will log the event to the logs channel.
a?maliciousaccountsettings <join/bots-added> status <on/off>
a?maliciousaccountsettings <join/bots-added> mode <mute/kick/ban>