What is this thing I found in my closet?

It’s not a cot, it’s small tree bugs. ;

More precisely:

Tree bugs with freshly hatched nymphs, probably the green ham

https://www.google.de/search?q=baumwanzen++nymphen&source=lnms&tbm=isch&sa=X&ved=2ahUKEwj_s_6GqtTpAhWGxqQKHTZYCEoQ_AUoAXoBIQAw&biw=1084&bih=601

https://www.youtube.com/watch?v=xir9DzSs6aA

Harmless little animals.

There a tree mother smuggled into the house and her eggs were placed in your closet. Now the little nymphs slip.

You don’t have to worry, because as the name reveals, these bugs belong to and into the trees, i.e. outside.
In the closet, in the house they find nothing consuming, so do not count among the pests!

Just collect and put it in the open.