Husband's little cactus has sickly looking (but not shrivelled) whitish/yellowish leaves with purple coloured main 'vein' up the middle (but not all over the plant). Is in a bright, northern window at work, regular winter darkness, doesn't appear over-watered, not rotting or soft at base. Can't find reference on net to leaf colouration. Never had problems w/ mine at home. Help please? Thank you!