Seems to be underwatered. I just hope your plant is in a proper container, with quality soil, and not some decorative gimmick... becuase it would mean root rot. The plant you have is called portulacaria afra variegata...

Annoyingly, overwatering would produce similar symptoms. Can you get the stones off the soil and investigate the moisture of the growing medium?

It really shouldn't be in a terrarium.

Oh. I didn't spot that! Yes. Bad idea. And as well as the humidity there is presumably no drainage.

I'll take it out and check the roots. And is there a way I can keep it in the terrarium? I mean, i can repot it if that's necessary but I don't wanna ruin the whole compossition...

Nope. Try bonsai pots with gritty soil. This plant grows somewhat fast for a succulent, and is used in bonsai. With a bit of work you may get it to look cool.

Terrariums are really bad for succulents, even if they look good for a while. Your plant was doing ok for abot a year, but maybe problems cougt up with it. Possibly, because there are drainage holes - without them, damage would, most likely, showed up much faster. Succulents are arid plants and need good air circulation (besides good drainage). Enclosed terrarium is much more humid than they like. Terrariums are terrific for plants that like high humidity and mositure - you can create great looking one with those plants. If you want good looking and healthy succulents, do them favour and grow them in more suitable conditions.

I took out the plant and the roots were fine, but it seems like they didn't have a lot of space to grow because they were touching the glass, so maybe that caused the problem? Idk, but I hope that it will get better soon!

The terrarium setup is wrong for a succulent. That’s the problem, as others have stated. It’s appearance is a message to you! Repot it appropriately, and get a tropical houseplant for the terrarium. Don’t let a tropical terrarium sit in the sun.

Forgot to mention it, but I repoted it

Let it get dry before watering, and give it your brightest spot that is not direct sun. It has too much space between its leaves because it needs more light. Terra cotta is best for this plant.

Agree with Laurie about mix drying up between watering. Did the roots look ok when repotting?

Plant doesn't really look etiolated, but very good ligth is needed. I have few in full sunlight - after being properly acclimatized to it.