Spider mites and other annoying bugs are no strangers to my plants. Luckily, lithops can deal with bugs very well. Even if their epidermis has been damaged by mites or roots have been nibbled on by mealy bugs it is not tragic and they get over it quickly. Also, it "helps" to have other mesembs around as those seem to be softer and tastier. Too bad that those are much less tolerant to bugs and it is difficult to get rid of them on such plants as Trichodiadema or Delosperma or Anacampseros or any other branchy plant with lots of hiding places. The damage also stays for what feels like forever. Once damaged and the plant survived it there's no going back to being pretty. I am in the process of getting rid of all those tasty plants and focusing more on lithops and conophytums. At a certain point it is okay to admit defeat.
But let's get back to lithops. If you notice that your plants don't absorb water when they clearly should be doing it (during the period of active growth) don't wait too long and check the roots. You will most certainly find mealies sitting there. Clean and wash the roots (it is ok to remove most of them in the process) and put the plant into fresh clean substrate. It will be as good as new shortly. New roots grow very quickly.
In case of mites (and similar biters), the result of their work is not as quickly removed. You do the same, clean and wash and transplant, but the damage will only go away with the next regeneration. But that's fine. Lithops growers are patient people. Even if you don't notice mites in time and the whole surface of the plant is damaged and covered in white dry shell the chances are still good that it will make it.
What happens is that the bugs start with the softer epidermis on the sides. Those parts are under ground and you might not notice anything at first. Keeping old leaves protects the plant to a certain extent but the bugs will find the spot above old leaves and beneath the top part. Once they are done with the sides they move up above ground. That's when you normally notice the damage done.
Some of my younger seedlings have been affected this year. All the sides and outer margins are damaged now. Some tops, too. See the typical white spots?
Hopefully next year it will be better for them. At least some of them are still looking pretty.
I have a great example of lithops that got over biting-bug damage in the past. I am very proud of them. The below are photos of the same plants, 7 years apart. I wrote about the damage here, here and here.