From the release notes of Slack for iOS, version 3.23.2:
Congratulations to the eagle-eyed winners of this week’s Spot the Difference competition, who noticed that we’d subtly updated the app icon, but had forgotten to mention it. You win our heartfelt admiration, and the pleasant, warm, tingling sensation of having been right all along.
The old icon was more playful with the lighter colors. I like the new one though.