The reason so many classic Disney movies are based on fairy tales is twofold. First of all, those stories are long beloved: Cinderella, Hercules, The Jungle Book, and The Little Mermaid have all survived centuries or millennia. They're the fabric that binds our culture to itself, the very stuff that dreams are made of.
The second reason is that all these stories are in the public domain. The public domain is a process that starts when an author dies. Essentially, a period of time passes and then nobody can own the work. If you wanted to right now make a movie called, say, Kill Bill, Quentin Tarantino could sue you. But if you wanted to make a movie out of Tom Sawyer, you're entirely free to do so. Nobody owns the work anymore, it's public property. For an unpublished work, that process takes 70 years. For published works, any story written before 1923 is fair game. Read more on the law here.
Disney took a ton of public domain stories and made them into now-iconic animated movies. The thing is that they don't now own the property. They just own their version of the property. If someone else wanted to go and make a non-Disney take on Hans Christian Andersen's The Little Mermaid, they are free to do so.
That's exactly what happened here. The non-Disney Little Mermaid stars Shirley MacLaine, Poppy Drayton from Downton Abbey, William Moseley from The Chronicles of Narnia, Loreto Peralta from Instructions Not Included, and Gina Gershon from Empire.
So, no, it's not the semi-troubled live action Disney adaptation, but it's still got some mf mermaids. Watch the trailer below. Looks pretty good.