How do i fix this rear cassette wobble? Deore 12 speed

The (mountain)bike is just year old and hasn’t been ridden very hard. This wobble developed in time and is now causing gearing and noise etc. issues. How can i fix it?

There’s also this tiny little thing sitting there. It’s as thin as a thin keyring. Can be seen in the video as well. Otherwise it’s just thru axle and no other part is missing afaik. First time taking the rear wheel out on this bike.

https://preview.redd.it/po6pqif6lz7e1.jpeg?width=960&format=pjpg&auto=webp&s=ef55ed4d81478c845ac536958f00933536dcacf3