Cactus became to decolor recently, mostly near the roots (not the yellow-green parts, talking about white-green-ish parts). They are not mushy, or hard as all normal green parts. Also, on the over side of the cactus, there are same patches but with something like white lines (like stretch marks?). Was watering it once in a 1.5-2 weeks.
As these parts are still hard I believe it's not some kind of rot, but I may be wrong as I'm fairly new to cactus-things. Thoughts were that this is due to too much exposure to sunlight so I moved in a little further from the window, however, it didn't help. It was moved to a new pot recently via transshipment, roots were looking good. Smaller plants near it (seen in the pictures) are growing nicely, they were transferred together with the big one.
What could it be? Is it dying and requiring some more or less urgent action or it is not as bad?