East Blue is one of three difficulties in the Treasure Log. This used to be known as Dream Log in previous iterations of the series, where you would go in with randomly assigned partners related to the groups involved in each mission. For this trophy, you only have to do one mission, though once you've defeated the Dramatic Log's challenges, there will be a plethora of things you've got to unlock that will be hiding under the Treasure Log's unlocks.
If you want to take it chill, try the Lv.1 challenges in East Blue; meanwhile, if you're well suited for the challenges that the end of the Dramatic Log put before you, look for any and all of the key missions. The key missions are vital to proceeding through the Treasure Log's invisible gates, eventually uncovering the Decisive Battle you need to advance to the next difficulty. Furthermore, some missions will unlock missions in higher difficulties simply by completing them, and you might need some of the gold coins that are hidden behind them.