Have you been using Slack on your Android phone? It is time that you change your password. In an email that Slack has been sending out to users, it has discovered a bug on December 21 that stored user passwords in plain text instead of an encrypted format.
No app should ever store critical information such as login credentials in plain text and instead save them in an encrypted format. Extremely simple to change your password as a Slack user. You should definitely update the app from the play store and then proceed with the instructions. Even if you have not received an email from Slack requesting you to change your password, it is highly recommended that you change your password as it may have reached the hands of hackers.
It is extremely strange that such a big company like Slack would make such a silly mistake.