When Amazon introduced their new calling and messaging features for Alexa devices, they selected a pulsing green light as the indicator of an unheard message waiting to be played. The issue with that is it’s very similar to the spinning green light that indicates a call is coming in, so it’s very easy to mistake one for the other.
Amazon will soon be resolving this by replacing the pulsing green message indicator light with a solid yellow light. Yellow is not used for any other Echo and Echo Dot light ring functionality, so it should be obvious that a new message is waiting, that is, now that you know what it means. If you see a yellow light ring, just say “Alexa, play my message” to hear the message. Amazon says this change will go into effect “in the coming weeks.”