Thanks Bill for your help! It's a 30cm x 30 cm mainly decorative terrarium for a 5cm pot plant, it's the only plant in it, but does not have the air flow indeed... Below is a picture of it from the producer, I will keep the main door open often to increase the air flow.
I have also taken a picture of the roots. It seems that there is a part covered in black residue. I know that the previous owner kept it in an aquarium, could it be some sort of algae or is it root rot? I repotted it into fresh moss with some perlite and very fine orchid bark meanwhile, in case the old moss was causing an issue (it was very wet as well).