Today, it is primarily through Poco Bueno's bloodline, say researchers at Mississippi State University and Cornell University, that the recessive gene that causes hyperelastosis cutis (HC) has passed
When a horse has HC, there is lack of adhesion within the dermis, the deep layer of the skin, due to a collagen defect. Collagen serves a form of glue that holds the skin layers together. In horses with HERDA, the "glue" is inferior and the skin layers separate.

Most likely not what she has, but easily ruled out with a Pedigree check or a DNA test.