The stock closed down 6.98 percent after trading lower for part of the day on Wednesday.
Snap, which operates a camera-based phone application that allows people to share photos and short videos, has proved a tough investment for many since its initial public offering in March 2017.
Since the IPO, Snap's stock is down nearly 60 percent, a slide many have blamed on lackluster engagement growth, slow monetization and disappointing innovation from Chief Executive Evan Spiegel.
Greenfield had been neutral on the shares since initiating coverage in April 2017.
While the Snapchat parent beat on both earnings and revenue in the second quarter, executives disclosed that the app's number of daily active users dropped to 188 million from 192 million.
It also issued guidance that fell short of analyst expectations.
While Snap closed Tuesday at $9.89 per share, the analyst believes it will slide to $5 per share by September 2019.
Others have pointed to the success of Facebook's Instagram as a major headwind for Los Angeles-based Snap. Since Instagram launched its Stories feature just over two years ago, it now has more than twice the daily active users of Snapchat, according to BTIG.
"We have been disappointed in Snap's product evolution (as have users) and see no reason to believe this will change," continued Greenfield. "We have not seen any meaningful innovation since the IPO; Snapchat has simply been out-innovated by Instagram."
Jefferies also cut its forecast for Snap, issuing a new 12-month price target of $11 per share Wednesday.
Echoing Greenfield's criticism, analyst Brent Thill told clients that early analysis shows persistent declines in user engagement in the third quarter.
"While still too early to call, Snap's flywheel of engagement seems to have stalled with both daily active users and time spent beginning to trend in the wrong direction," Thill warned. "Snap's position as a communications platform and content distribution platform hinges on the need for users to have multiple friends on the platform."
"If users begin to churn to other services, it could cause a negative flywheel as users have less incentive to open the app," he added.
Thill said he reduced his 2019 revenue estimate to reflect his growing uncertainty around user interest. He now sees income at $1.53 billion for the year.
Disclosure: CNBC's parent company, NBCUniversal, is an investor in Snap.