Desktop version of Slack is based on Electron, which is basically standalone Chromium (open-source core of Google Chrome) plus some host OS integrations. In other words: it's a separate web browser, but limited to Slack.
Bad news: Electron is a known resource hog. There is no way to fix it for you as a user. Slack developers are probably aware of this issue for some time now and they didn't fix it (and fixing it wouldn't be easy).
Good news: It's not really consuming 1 GB of RAM. Chromium's sandbox uses multiple processes that share some memory. So when simply summing per-process values, you're counting some memory areas multiple times. Still, it's probably using well over 500 MB.
One way to reduce Slack's memory consumption would be to use it in a browser instead of the standalone app.