Hi,
It is a warning stated that ssl certificate was issued for another address than used to access site(i suppose that you use default certificate that is generated during Plesk installation).
If you want to get rid of this warning you need to create you own self-signed ssl certificate(or CSR and send it to the one of the certification authority in order to generate certificate signed by CA) with settings that you need and install it as Plesk CP certificate(all these can be done at Plesk > server > certificates page).