According to the picture, the "app" you called is actually a part of the Steam app, you should first make sure the app is not open on your Mac and then head to delete the Steam app straightforwardly and its leftovers.
You can open the Apple menu by clicking the Apple logo at the top left corner and choosing Force Quit to check all the running apps on your Mac, check whether the Steam app is in use, if it is, select it and click Force Quit, and click Force Quit again to close the app.
Then you can proceed to uninstall applications on Mac from Finder, Launchpad, Terminal, or via a third-party uninstaller for Mac such as iBoysoft MagicMenu. In case Apple's solutions fail you, you can try iBoysoft MagicMenu to uninstall apps on your Mac within one click.
After deleting the app from your Mac, you should thoroughly remove its leftovers so as to release more disk space on your Mac. You should navigate to the ~/Library/ folder in Finder and delete the associated files. Alternatively, you can download a Mac cleaner to clear junk files with ease.
iBoysoft DiskGeeker is a good choice for you. The clean junk feature aims to check for redundant data on the system drive and can locate data and files that are no longer required in the system cache, user logs, application cache, downloads, etc.