If you would like to generate an MD5 that is consistent with other databases (which take a string parameter for its MD5 function) you can do the following:
Within Athena: select to_hex(md5(to_utf8('hello world'))) returns: 5EB63BBBE01EEED093CB22BB8F5ACDC3
Within PostgreSQL: select md5('hello world') returns: 5eb63bbbe01eeed093cb22bb8f5acdc3