Mail.Message ,How to convert it to String

You can use the Body property of the MailMessage type to get the mail message content as a string.
If you have the output of a Get Mails activity in a variable, let’s say myEmails and you want to get the content of the first email, you can use: myEmails(0).Body
There are other properties available, like Subject, From, Header, etc. that can be useful.