I have a Phal. that has looked like this for months but is slowly turning back... All of the leaves on the one side looked like that. I've given it a little less light. It has been darkening up again but very slowly. I received it that way this year and it has shown no decline whatsoever in the months I've had it.
If it were chlorosis due to magnesium deficiency it would start with the old growths... And you mentioned the new growth being affected.