Elizabeth Stuart and not Arabella Stuart as stated on the plaque which was added at a later date
Her white skin makes me wonder if she had rickets somewhere during her life or was this white make up (that could have been made with white lead pigment).
From www.kleio.org/en/history/famtree/tudors_stuarts/abb200v where Herr Maike Vogt-Luerssen disagrees with the caption on the portrait. According to Yale University, this could be "One of three miniatures in watercolour on vellum, set inside an ebony veneered 17th or early 18th century case with inner gilt slips." However, this particular version of the portrait is shown as a full-size image here. The three miniatures are presumably derived from this portrait that could also be a Hilliard. This portrait is located at Hardwick Hall, Derbyshire UK.