Does green bubble mean phone turned off

Green bubbles indicate that the message was sent using SMS instead of iMessage (blue bubble). This could be for many reasons, such as the recieving iPhone not having a data connection.

You can disable the SMS fallback by going to Settings > Messages and turn off the "Send as SMS" option.

Green means that it is a normal text, blue means that the message got sent as an imessage. If it switched from blue to green it may mean that the imessage could not be sent so it was sent as SMS

It means you lost a data connection at either end (WiFi or cell data), and the message had to be sent over the SMS voice channels.