by my mikrotik SN i get x= as ASCII 
So i think, i have to use HWTC and then the SN of the SFP.
But i think i found a Solution (but i have not tested yet) here:
http://forum.mikrotik.com/t/how-to-connect-via-ssh-to-gpon-spf-module/161880/1
I have bought a SFP with ONU and MAC, thanks to @nFin1ty fr this information!