why the text bubble is green when send to iPhone

Hello,

If you see a green message bubble

If you see a green message bubble instead of a blue one, then that message was sent using MMS/SMS instead of iMessage. There are several reasons for this:

• 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.

Learn the difference between iMessage and MMS/SMS messages.

From:

If you can't send or receive messages on your iPhone, iPad, or iPod touch - Apple Support