This text is escaped JSON, which means you’ll have to either parse it as JSON and then get the value of the ‘message’ key - or you can simply manipulate this text by getting rid of the unwanted text and retaining the contents of the message.
I’d go with option 2 if this format isn’t going to change.
I’d use the :split by operator on this text twice:
first, split by "message\":\" and on item#2, split it by \",\"payload\" and take the first item. This will be your message’s content.