MacOS – What app is this icon with a square and a black triangle

You might want to make sure your app is correctly associated (for that you can use the free RCDefaultApp panel).

If you fancy changing icons, you can try the arguably pricey Candybar from Panic inc. which is excellent if you love playing with your icons, but that's all it does, so for 6 Starbuck's Coffee Latte you can do that :p

Please note that if you change an icon, you will probably have to restart the finder (and/or the Dock) for the change to be reflected (Logoff also works).

Turns out, I just had to reset the 'dock' process. I opened Terminal and typed

killall Dock

and pressed enter. The screen flashed for a second, and everything came back with my stuck icon gone.

If that fails, the issue may be with Finder (this is the case with the similar issue of Stacks categories "burned in" to your desktop background). Running

killall Finder

will address that.