It is a predatory Lithobius forficatus. Probably got into the terrarium with moss or rotten wood. They are not dangerous for an adult Mardonius parilis, but they can eat babies whose chitin has not yet matured and who still do not know how to defend themselves.
Marysia is a very beautiful cat with tassels on her ears and a marble pattern on her back. She will live with us for a while, and then she will return home again.