Is there a way to hide the Crop-Folder for images for the users, who just edit content?
Can I move is outside the media source folder or add an . before folder name, like .crops/?
One thing you can do is configure the skipFiles property of your media source to include the crops directory. That will hide it.
You can also change the crop directory on the field properties to something else, however, this does not automatically update references to it so that might break file URLs if you have existing content using hte crops.