The green message bubble indicates the message was sent as an MMS/SMS instead of iMessage. Some reasons you might see a green bubble is...
- The person that you sent the message to doesn't have an Apple device.
- iMessage is turned off on your device or on your recipient’s device. To check if iMessage is turned on for your device, go to Settings > Messages > iMessage.
- iMessage is temporarily unavailable on your device or on your recipient’s device.