A passport is needed if one flies from the US mainland or Hawaii to Japan then Guam.

A passport is good to carry if flying from Hawaii to Guam in case of diversion though it's very unlikely that it would divert to any foreign country.

Guam is US territory but some proof of citizen is useful because people from Red China go to Guam without a visa and the U.S. is afraid they will sneak off the island. No joke.