Please confirm that you are using the Messages application and it was not a SMS message. On the top of the conversation where you see the message, if it says "Text Message", then it is a SMS message and the only legitimate Apple Cash transfer would be from one that shows "iMessage".
It may be the case that you have to verify your identity. This can happen more than once, but usually you will see a message indicating that when viewing your Apple Cash card in the Wallet or Settings. Does the transaction show in your Wallet with any attached message?
Verify your identity for Apple Card or Apple Cash - Apple Support
In all cases, you can contact Green Dot Bank by calling (877) 233-8552.