You can call the Character Meet and Greet phone line from your hotel room to hear a recorded message with times and locations for all the characters, including Moana.
As others have mentioned, Moana also appears briefly during the first two storytellings at the fire pit (near Ama Ama) to help Uncle tell stories about Maui.
In addition, Moana makes a surprise appearance during Uncle's Kahakai Canoe Racing, which is a free Ohana activity that you can sign up for in the morning at the Pau Hana Room. (Check the Daily Iwa for times.) The activity involves building and designing a small wooden model canoe as a family, and Moana comes in to greet all the new wayfinders, inspect each family's canoes, and take pictures with everyone. Moana then leaves and everyone takes their new canoes to the lagoon to race. It was a really fun way for us to meet and interact with Moana and came totally unexpected!