Morgan Stanley cited mixed second-quarter results, increased competition and a weaker IT spending environment for the cut.
"While the long-term opportunity remains, we do not think Slack will benefit from expanding multiples, so upside from current levels will come from more gradual outperformance over time," analyst Keith Weiss wrote in a note to clients.
The analyst also said competition from Microsoft's Teams product "will limit Slack's realizable TAM," or total addressable market.
Weiss reiterated his equal-weight rating on the stock. Slack has eight buys, eight holds and one sell with an average price target of $32, according to Bloomberg data.
The stock was trading down 7.64% to $22.85 in trading Wednesday.