What is This Exclamation Mark on the Club Chat Messages?

With update 2.1.58, you now have the option to Report a club member's message if you find it offensive or otherwise inappropriate. If a number of people also report the said club member, he or she will lose the ability to send a message to the club chat for a period of time.

Club leaders cannot be reported. Also, there will be no exlamation mark on your own messages since you can't report yourself.

Once the exclamation mark button is clicked, you will see the message has now a "Report Message?" bar right above it. Using this bar, you can either click "Report" and report the chosen message or click the X button to cancel reporting.