When I click on it it does not display anything, but it actually popped up after I installed Dropbpox ...
You can install the icon for Dropbox manually. Check here
Make sure you have git installed via
sudo apt install git
git clone https://github.com/zant95/elementary-dropbox /tmp/elementary-dropbox
The icon itself means that there's a program running and the icon specified in its .desktop file cant be found in any of the icon locations.
Its a common thing with Dropbox for example (if its indeed dropbox let me know if you need a fix) Other programs might have this behavior as well.
try clicking it and see which program's menu opens