Even though you can meet Ariel from the live-action film, The Little Mermaid, you can still meet Ariel from the animated movie at the Royal Hall or anywhere else she spontaneously appears. Want to know a secret? You may know that you can check the times that princesses are in the Royal Hall in the Disneyland app.
Meeting MoanaMoana can currently be met in Adventureland at Disneyland (DLR), and during special ticketed events at WDW (MNSSHP and MVMCP). At Disney Aulani Moana can be found around the Waikohole Valley intermittently throughout the week.