The slack "app" is just their web site and a web browser zipped together in such a way that it doesn't actually quit when you tell it to quit. Why anyone wants that I cannot begin to imagine.
And there's an option to turn that behaviour off, if you don't like it.
But I would prefer if it wasn't all Electron because it really does feel like the resource overhead is unnecessary. But if it wasn't done with this tech, we probably wouldn't have a Linux client at all, and maybe not a Mac one either, and personally I prefer it to feel like an application instead of having to run it inside a browser.
Kind of hoping Flutter helps advance the situation of cross-platform desktop/web things in a positive way.
When you Command-Q Slack, it should close completely (which afaik it does).
To quit it, choose that from it's menu or use the cmnd-q shortcut IIRC.