Q: Why are my iPod text bubbles green?

It means the messages were sent as SMS vs iMessage. -AJ

Probably because it's linked to your phone via continuity.

You may be using a feature called continuity. See this support article for more information. Use Continuity to connect your iPhone, iPad, iPod touch, and Mac - Apple Support

SMS

With Continuity, all the SMS and MMS text messages that you send and receive on your iPhone can also appear on your Mac, iPad, and iPod touch. You can also reply to your contacts from whichever device is closest to you, including your iPad or Mac.

You can start a conversation in the Messages app or click a phone number in Safari, Contacts, or Calendar.

Set up SMS and MMS with Continuity

1. You need iOS 8 or later on your iOS devices and OS X Yosemite or later on your Mac.
2. Sign in to iMessage with the same Apple ID on your iPhone, your other iOS devices, and your Mac.
3. On your iPhone, go to Settings > Messages > Send & Receive. Add a check to both your phone number and email address. Then go to Settings > Messages > Text Message Forwarding and enable the device or devices that you want to forward messages to.
4. Look for a code on the Mac, iPad, or iPod touch that you enabled. Then enter this verification code on your iPhone.