Even if a particular moderator is shown as online and another is not, when you have a problem to report that requires a moderator (a rules violation, a thread in wrong forum, commercial advertising, etc.) please use the Report Bad Post feature - that
icon - rather than PMing a particular moderator.
When you use the Report Bad Post feature, the message is seen by whichever moderators are on duty. Private Messages are not always seen as promptly.
If you have something to say to a particular moderator, then a Private Message is fine.