Not Duh!
it's whiter and brighter than the stuff around it, but that doesn't make it white, per se.
Standard optical illusion territory here.
problem is, the white rock is surrounded by particularly dark dust fields.
If you take a map of the whole globe of Mars and cut out the bit with the white rock in it, then move it to somewhere else whare the dust is not so dark, it matches neatly for colour and reflectance to the standard tan surface.
Try it, the results may surprise you...