In Neil Gaiman's novel, Stardust, Morwanneg is an alias used by the Witch-Queen of the Lilim. When the Witch-Queen encounters Ditchwater Sal, aka, Mistress Semele, she used this alias. In the novel, the name was defined as "wave of the sea". The Lilim were once beautiful queens of a magical kingdom that was lost beneath the sea, and it is stated in the novel that she used the name because her true name was drowned and lost beneath the cold ocean. In the movie, the character was called Lamia, but in the novel her name was not given.
The name was a Welsh sound to it, but I cannot find it in any baby name databases. Perhaps Gaiman created it from Welsh elements. It does not appear in the Social Security name database.