Have you watered or sprayed (with water of course) the plant recently?
I think it could be a water retention problem:
Sometimes, when you water the plant too much, those spots can appear, I'm not actually sure why, if I'm not mistaken, it's the veins in the leaves that pop, (don't quote me on that). Anyway. that could be it.
Although I have to point out that it can also accrue without over-watering, I've seen plants do that even with minimal watering.
Of course, there could be other reasons, but I'm quite confident that I'm right. Do let us know