It's part of the May 2017update which keeps the copied item active in the clipboard even if you do things that would normally clear it, like typing in a cell, applying formatting, etc.
Pressing ESC will clear the marquee/dancing ants.
What a terrible change! I can see why they did it: trying to have "Persistent Clipboard" like the other office programs so you can keep items in the clipboard without having to copy them again and again. This becomes a pain in excel, however, since more
often than not you want to just copy and paste the range once and move on. Having to constantly clear the copy range with esc key will be way more of a pain to most heavy excel users like myself. They should at least give us the option to turn this feature
on or off.