Hi. I hope this is right place for the question and that it's not a duplicate I am concerned by the white spots as shown in the picture. What can that be? It affects only one of two plants sitting in the same pot.
It appears to be a bad case of mealybug infestation, though it's hard to magnify your photo sufficiently to see them clearly - if it's not on your other plant, it might already be present, but not enough for you to notice yet, so check carefully, they may all need treatment. Insecticidal soap spray should help to clear them as it's so widespread, otherwise, touching the bugs with a cotton bud soaked in alcohol will kill them for smaller infestations. http://extension.umd.edu/hgic/topics/mealybugs-houseplants