Does that little pot have a drainage hole? And even if it does it's too small. Also the soil looks as if it retains moisture. Repot in a fast draining mix in a larger pot with drain holes. Water sparingly and give a lot of light.

Ps Has the cactus in the tin can got holes?

PPS containers with incurving rims are a nightmare for repotting. You'd do better with a flared opening.

Yes, they all got holes (I made them in the tin can).

So do you think it's only a problem of pot?

It's a problem of potting mix. Cacti and succulents want a very gritty potting mix with almost no organic matter (bark, peat moss, regular houseplant potting soil) but lots of granite or poultry grit, perlite or pumice or calcined clay, like Turface. And they want very little water during the winter months. Check them monthly but they may not even need any then.

Red specks - might be spider mites.