This email from Slack is real, change your password if you receive it (Source: Android Police)
If you use Slack on your Android phone, you need to change your password now. The company sent out an email to some users requiring them to reset their passwords. According to the company, from December 21 to January 21, the Android version of the Slack app would store user credentials in plain text. The Verge points out that, in theory, this means any other app on your phone could access them.
Before changing your password, make sure to update to the latest version of Slack first. And if you received the email from Slack, click on the link to change your password. Otherwise, you'll need to be on your desktop or laptop to reset your password. Slack has instructions up on how to do that. If you use your password on other sites (which you really shouldn't do), make sure to change those, too.