What are these white marks on my Lithops? (mould, disease..)

[Greencoffeegirl]

Hello,

Can anyone help me identify these white areas that started showing up on the base of my Lithops?

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These Lithops are fairly new to me (about 3 months). I bought them at a large plant nursery, and planted them in a terrarium containing a mix of a lot of perlite, gravel, and sand; and a tiny amount of coir-based potting mix. It’s very well-draining medium, but I haven’t even watered them once yet.

I didn’t notice the white marks immediately when I planted them. I noticed them about a month and a half ago. They started a bit smaller, and have grown to this size over the last 1.5 months, so they seem to be growing albeit slowly. I’m also starting to notice them in other areas around the base of this plant cluster now.

The terrarium receives plenty of light (south-facing window + grow light when needed); and plenty of air circulation from a fan.

So, what is it? And should I do anything about it, or just see if it clears up on its own?

Thank you in advance!