The value we wanted should have been a simple string, and we did not
 know how to convert the output to string
##class(%SYSTEM.Encryption).MD5Hash() does return a string, what do you mean by 'simple'? If you mean 'displayable' that will depend on what you're trying to display on.
Take a look at ##class(%SYSTEM.Encryption).Base64Encode() that will return a string with only non-control ASCII characters.