Slack.Lookups.lookup_user_name
You're seeing just the function
lookup_user_name, go back to Slack.Lookups module for more information.
Turns a Slack user ID ("U…"), direct message ID ("D…"), or bot ID ("B…")
into a string in the format "@USER_NAME".
NOTE: Referencing "@USER_NAME" is deprecated, and should not be used.
For more information see https://api.slack.com/changelog/2017-09-the-one-about-usernames