Thank you so much for your answer !!!

I was so worried !! So the hand sanitizing-water mixture isn't going to be the death of my succulents ??

That looks like peat pellets - is it peat?
If yes, it may hold too much water or be completely dry. Can you find some perlite? How about some grit - size approx. of aquarium gravel. Are you in USA or Canada? (I don't think so but don't want to assume...)
Rina

Nope, I'm from France :) I'm still new to this whole succulent world, but I think they are called pellets, you just add some water and it absorbs it and becomes soil. I will try to find perlite, i don't know where yet, but i will look around, and when i get it what should i do with it ?

Here is how the soil looks under the rocks

And that is the "nursery" , it's the same soil, I hope you see what it is

Yes, that's exactly what I was expecting, they are know around here as peat pellets (that's why I asked where you live-& the name on container was a clue too). It was probably too wet, and that's why bugs like it.
It could be difficult to keep peat watered 'just right'. If wet, it very easily becomes too saturated & could cause root rot, especially for succulents. Very easily it turns into a peat soup :) If it dries out, it could be as hard as rock. That's why it is not recommended for growing plants. But it is OK for seed starting, or even rooting the leaves as you did above. Don't byrry them, they are growing roots into it. As they grow bigger (the babies), it would be better to pot them up into well draining potting mix.
Perlite and/or grit could be mixed with some soil. If you could find cactus & succulent soil, add at least 50% of perlite and mix well. That would be much better to use for your plants. And the stones you have are nice top-dressing, just as you have it. Could you find some fine gravel? It would be good to mix some with perlite if available. (Perlite should be available in any garden center, or some other stores that carry gardening supplies and plants).

To check if plant needs watering, you can use wooden skewer. Insert at least half way into the mix. If it comes out with particles sticking to it, no need to water yet.

Rina

I normally use a spray bottle to water them, so the soil doesn't get soaked. But i will Go out and ask where I can find perlite here :)

Thank you so much for your answers !! You helped a lot !

Hand sanitizer doesn't contain soap. I don't think it'll give you the best results. A little dish soap is a better choice.

To kill mites, I used a little dish soap and water, and added a wee bit of alcohol. I don't know whether the alcohol was needed, but the mixture did a good job on the mites and didn't damage the plants. You'll probably have to spray every X number of days to kill all generations as they emerge. I don't know the lifecycle of aphids, but many pests have stages that are resistant to treatment - eggs, for example. You'll have to look-up the lifecycle of black aphids to find the vulnerable stages unless someone here knows.

I added the hand sanitizer, for the alcohol it has in it ^^

But thank you for your tips, I will try the thing with soap :)

I think the soap is added to make solution little more 'sticky' - correct me if wrong (and just a drop or few are needed).
Alcohol solution works on many bugs. Scale is easily wiped off/killed with cotton ball soaked in alcohol, or q-tip. But if there is many bugs, spray is better.
Just don't use too much of soap or hand sanitizer (what else is in it?)

It's not like the american-scented types don't worry, i'm stupid but not enough to do such a big mistake ^^

it has things like alcohol, isopropyl alcohol, glycerin :)