Hardware Monitor is also available in a “Lite” version. The Lite version can display readings in the menu-bar of macOS without needing additional windows or consuming valuable screen space in the Dock.
You should have set the menu-bar preferences in the “big” version of the application before launching the Lite version. To quit the Lite version, select the last menu item in the displayed menu. By selecting the second-but-last menu item you can launch the “big” version, for example to change preference settings.
If you disable the menu-bar display in the “big” application, the Lite version will only appear as an icon in the menu-bar, using the smallest screen real estate as possible.