Little green men - D.W Smith

Roswell is struck by what seems at first to be an unidentified virus. When Sheriff's son Kyle Valenti finds that his skin is turning slowly but surely a worrying shade of green, Liz drives him to the nearest hospital to have the cause checked out. There they find that Kyle is not alone: the town's medical facilities are overwhelmed by people reporting the same mysterious symptoms.
What's more, Max, Michael and Isobel are unaffected, which isn't such a good thing as it might appear. As more and more of Roswell's human population gradually succumbs, the military quarantines everyone whose skin shows even a trace of "alien"-looking green - leaving the pale-faced true aliens totally exposed.
Then Liz's research reveals that the cause of the epidemic is a cache of alien crystals near the crash site which have leached into the water supply. They destroy the natural pigementation of human skin... and who on Earth knows what permanent damage they could do...?