Every bus that goes to Limenas - Theologos stops at Potos, so you can catch those buses. There aren’t any other cheap options except renting a bicycle. Taking a taxi is also an alternative.
There should be more buses in July and August for Alyki to Potos. We will update the bus schedules shortly.
There isn’t a bus that goes directly to Keramoti from Thessaloniki airport. You can hire a mini bus or taxi to pick you up at the airport and drop you off right at the ferry in Keramoti.