It looks like you maybe created a widget?
Rt-Click on your desktop and enable “Customize Layout”
If that thing is a widget as I suspect … then mousing over it will then give you easy delete and other options.
“Activities” maybe ?
I have disabled those buttons, but the defaults for the key combo were Meta+Tab, Meta+Shift+Tab.
Using those to pop it up and then using Esc after interacting with it should allow it to close.
(by the way … if none of this works out … have you simply logged out or restarted?)