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You can make the trip as long as you return to Spain while your passport is still valid. There is no extra passport validity requirement for internal Schengen travel. To avoid complications, you should probably avoid itineraries that require a layover outside the Schengen area.
The Schengen area has a requirement for short-term visitors that passports be valid for three months beyond the anticipated departure from the Schengen area. But this is enforced only on entry into the Schengen area, so it does not apply to you. Furthermore, while you are a short-term visitor to Greece, you are not a short-term visitor to the Schengen area.