Do spider mites only attack one kind of cactus? The cactus is actually in a pot with 2 other kinds (again, cactus newbie so not sure what kind), but the others seem fine.

I'm fairly new to cacti growing and so I'm not really sure what is happening. I've had this cactus for a year and got it as a nice decoration, but I recently left it with a friend for a few months while I was away. When I came back, there were sort of brownish patches all over the cactus. I've resea...