You are right that your AIS is working fine. On the Raymarine C Series chartplotter there are four different conditions that the AIS icon shows. There are two icons that you would normally see. If the AIS is Off, or not connected, you will see the “No AIS” icon (greyed out AIS triangle). If the AIS is On and the chartplotter is set to default, you will see the “AIS” On icon (AIS triangle).
The Red "X" icon that you are seeing means that the "Dangerous target" alarm is off, but the AIS is On. The icon is really a Bell with a red "X", but the "X" makes the bell hard to see. You can turn the AIS Alarms On or Off in the Alarms Setup Menu. We recommend keeping the AIS alarm on. The last icon is the Bell by itself, which indicates that there is an active AIS alarm.