The Little Mermaid is not a Disney creation, it is a story by Hans Christian Andersen, as is The Snow Queen which Frozen is (very loosely) based on. You can't

The Little Mermaid is not a Disney creation, it is a story by Hans Christian Andersen, as is The Snow Queen which Frozen is (very loosely) based on. You can't trademark a story or idea, but you can trademark a particular character design. You can make your own Cinderella interpretation as long as you don't copy the exact art and design made by someone else.