Setouchi Triennale passport

Do you need a Setouchi Triennale passport if visiting Naoshima Island and Teshima Island during the Setouchi Triennale or are gallery entry fees separate?

A passport is not needed. You can also enter with regular tickets.
Two of the best museums, the Chichu Museum and the Teshima Art Museum, aren't actually even covered by the passport.

You need (or are highly recommended) to get a reservation for each of the museums. As I needed to learn the hard way last week, these reservations sell out.
Look up how early you can reserve and go for it.
I got a reservation for Teshima a few days before I wanted to go and was able to cancel it shortly before actually going (because weather seemed to be bad and I didn�ft want to get stranded on an island with no boat back).