I have noticed that a good amount of the leaves on my Pilea have started to discolor around the edges and in spots in the middle of the leaves, its not a browning or a yellowing, more like a translucent lighter green. I started noticing this a few months ago during the winter, none of the leaves affected have fallen off or died back. The parts of the plant that are affected are not soft to the touch at all. It also looks like the new growth on top is not being affected. As far as I can see, there are no visible signs of pests attacking my plant.
As for how I care for my plant:
I water about once a week, or when the top 2-3 inches of soil is dry
I fertilize with liquid fertilizer (following dilution directions on the bottle) during the spring and summer, so far I have only fertilized it once this year.
I keep my apartment between 64 to 74 degrees throughout the year (I am in zone 6b)
Unfortunately, I have a north-facing window so I have low light in my apartment, never getting direct sunlight.
So my current hypothesis is that it is discoloration from cold. I keep my plants in the brightest location in the apartment which is right next to the sliding door. And since my apartment is a bit outdated I feel like during the winter months (when I first started noticing this) the leaves were close enough to get some frostbite. Its also worth noting that my sister's Pilea is doing the same, it was a pup from this plant, she also keeps it near a window, but it is a better insulated one than mine.
I have searched high and low on the internet about this issue and I cannot find any definitive answer and I would love to hear anyones thoughts or ideas.