This is a quarantine of some extremely bright PNG files. They render brightest on Chrome-based tools with MacBook Pro screens.
If you know how to use the Terminal, run this to restart Slack without any HDR emojis:
killall Slack ; open /Applications/Slack.app/ --args --force-color-profile=srgb
(HDR emojis will return the next time you relaunch Slack.)
It's the same as above, though I needed to delay relaunching for some reason.
killall 'Google Chrome' ; sleep 3 ; open /Applications/Google\ Chrome.app/ --args --force-color-profile=srgb
HDR emojis will return the next time you relaunch Chrome. To permanently turn off HDR images in Chrome, paste this address in your browser's address bar:
chrome://flags/#force-color-profile
Set the Force color profile setting to sRGB.
Corry Haines has a helpful guide on how to create HDR emojis.