Facebook Chat on Iphone with little moon?

The question is, why do people that use Facebook messenger on their Iphone appear with the little moon next to their name? I have noticed that my boyfriend is either with the little moon while we're chatting on Facebook or nothing appears next to his name. Does this little moon appear only when he is actively chatting or could it be that when he is using Facebook and not the chat?

Moon Means They Are Idle