Hi, as far as I know there’s no discount on ferries from Turkey to Greece (only for some inland ferries in Greece).
But so or so at the moment ferries between Turkey and Greece are either not operating or only taking trucks on them (and no touristic passengers). 
And keep in mind that also the train from Sofia to Istanbul is suspended until further notice (last summer did not operate at all!).

The ferry from Cesme to Chios is not included in your pass. On the Chios - Pireas ferry you get a 20% or 30% discount, so you don't need to use a travel day. 

A few notes about trains in Turkey and Greece: as Martin pointed out, the night train to Istanbul is suspended because of covid. This also applies to the Sofia - Thessaloniki train. Keep yourself informed on the website of BDZ to see the current situation. The regional train Kapikule (at the Bulgarian border) - Edirne - Istanbul might be running, but it's hard to find up-to-date information.

Turkey has only recently resuming their regional and long-distance train services. Not all trains might be running yet. The high-speed trains to Konya and Ankara do run, but on a limited schedule. Check the current situation on site.

Hi, is there any train Athens to Istanbul?

Thanks!

No there is not, but there are busses which are not included in Interrail. You have to pay extra.
Maybe more beautiful will be to take a ferry to one of the Greek islands (e.g. Chios). From Chios you can take a ferry to Cesme and then from Cesme by bus to Izmir. From Izmir you can use Interrail again - there are trains to Eskisehir and Ankara (and also Konya, but if you are not religious, then better not go there). Also there is a train to Bandirma, from where you can catch a ferry to Istanbul.

Would suggest you this route instead of the loooong and boring bus trip.