Aegean Airlines has a web page about transiting/connecting passengers at the Athens airport. Under Extra Schengen to Extra Schengen:
If you fly to and from airports outside the Schengen Zone, after
passing through passport control, head to Gates A1-A23.
That seems to imply that you will enter the Schengen area, if only briefly.
You should be certain that the airline is correct about going through passport control during the transit. Find out which parts of the airport your flights will be using and whether it is possible to pass from one to the other without clearing passport control.
If the information is correct, then you will need another Schengen visa or to book a different ...
A single entry Schengen Visa may not be used to enter Croatia
a dual or multiple entry is required
MVEP • Visa requirements overview
Third-country nationals who are holders of:
uniform visa (C) for two or multiple entries, valid for all Schengen Area Member States;
You may be refused to board the plane in Athens to Croatia, since you do ...
This is related to My passport was stamped with an exit stamp while transiting to another Schengen country via Turkey. Was this a mistake?
You should apply for a double entry Schengen visa. This allows you to enter Greece and exit it two times even if only for transit. This will be suitable for both your transit and visit.
As you leave the transit areas ...