I just want to point out that there are actually two different Jane Yolen works that might get confused -- I was confusing them myself until yesterday.

"The Ballad of the White Seal Maid" is the poem which Lui Collins set to music.

"The White Seal Maid" is a short story (though quite a bit longer than the poem) from her 1983 (or perhaps 1987) collection, Tales of Wonder, republished in 2015. It's not simply a longer version of the poem; it's an entirely different story about two new characters, one of whom is a white seal maid and the other of whom is a fisherman.