That's still a lot of RAM for a chat with giant emojis.
You have to consider that people reporting these issues with memory consumption are running the app in different operating system than yours. Maybe your system is optimized. I just checked my co-workers computer and his main Slack process is at +97MB but there are 3 instances of "Slack Helper" at +433MB, +297MB and +255MB respectively. That's over 1GB in total. So either you are just looking at the main Slack process (which at +340MB makes sense) or your Slack is leaking memory somewhere that your RAM monitor is not able to map.