Welcome To HealthcareMagic forum,
Simon ,I need to know whether this white mark is raised above the skin level or flat .Is it milky white .I would also like to know your age and other systemic illness if any.
I can give you two possibility with little information you had given.1) vitiligo
(if lesion is milky white and flat).
2) xanthelasma (if raised and rough).
For further information I would require above details.