I’m not sure what is causing the marks, but it’s fairly normal for mesembs to get marks on their epidermis. I don’t own this plant, but own other mesembs. I’ll try to help🙂. How much are you watering, how much light/sun, what’s the humidity like, and type of soil?

How long have you had it?

My first impression is in line with Kara. That looks more like scuff marks that have healed rather than marks caused by a disease. However, I've not had a lot of experience with these.

Technically, they aren't Lithops but you do have the correct name Argyroderma delaetii. Looks like the Argyroderma are lumped in with the Lithops in the way that they are both often called living stones. They're in the same family, Aizoaceae. I mentioned all of that in case someone wants to try searching Google for info about the plants in question. Might help sort out the info that applies to the issue and what info doesn't. Though just now I tried searching for Argyroderma disease and didn't find much that was helpful. Seems like the biggest concern with growing these is overdoing the watering and causing leaves to split.

I guess if it were mine and I was sure there was an infection I'd try spraying it lightly with some Bayer 3 in 1. However, I'm not sure what that might do to the skin of the rest of the plant. Some topical sprays have been known to cause problems for some cacti and succulents.

I have this plant - looks to me like marks from stretching from slight overwatering. Hope this helps.

You can see I had this happen to me too :)

I was thinking those are healed scuff marks but now I've noticed there are some marks between the leaves where scuffs seem unlikely to happen.

do you fertilize them? they look like burn marks