Assuming that the risk could be a MiTM attack between your Spring client and the REST service, it doesn't necessary matter whether it's in a query string or POST body. A query string would get cached (in a browser, proxy server, ...) which could lead to disclosure; but all in all I would encourage using end to end encryption.
If you Spring client ensures that it's talking to the right REST service (cert validation, pinning, ...) - then you can send "secrets" over that connection with a reasonable assurance of confidentiality and integrity.
Additionally, I don't know what your "encrypted" value is, if it's encrypted with a shared key, then someone could spend enough resources/time to reverse engineer it and get the secret. If you're sending secret data; try to stay away from custom implementations; an end-to-end SSL/TLS tunnel has been proven very robust.