On Monday, in a series of tweets, Musk claimed Apple has “mostly stopped” its advertising on Twitter. That’s bad news, as Apple’s $48 million worth of ad spending accounted for more than 4% of Twitter’s revenue for the first quarter of this year, the Washington Post reported. 

“Do they hate free speech in America,” Musk wrote. In a separate tweet, he asked Apple’s CEO, “what’s going on here @tim_cook?”

Twitter has experienced an advertiser exodus since Musk took over last month, and Apple is not the only advertiser to head for the exit. Other major companies including Volkswagen, Pfizer, and General Mills have paused their ad spending. And not to mention that since his takeover, Twitter’s lost half of its top 100 advertisers, according to a report from Media Matters. Musk has previously blamed the “massive drop in revenue” on activist groups pressuring advertisers. 

But for all of Musk’s fighting words about Apple pulling its Twitter ads, there’s a much bigger problem looming over the relationship between both companies. 

Musk tweeted on Monday that Apple has “threatened to withhold” Twitter from its App Store. Apple has not confirmed this, but there has been widespread speculation that such a decision would be related to Musk’s moves to gut Twitter’s content moderation team, something that human rights groups have warned risks making the platform party to harassment and abuse. 

Apple is a major arbiter of content on the internet by the way it decides which apps are available to iPhone users. A few years ago, Apple was reportedly considering kicking Uber off the App Store for breaking Apple’s rules. If that happened, it would have effectively destroyed the company—people wouldn’t be able to call for an Uber on their iPhones. 

Twitter’s former head of trust and safety, Yoel Roth, who quit soon after Musk took over, wrote an op-ed for the New York Times outlining what could happen at a post-Musk takeover of Twitter, particularly in terms of Musk’s self-proclaimed devotion to “free speech.” Along with advertisers and regulators, Roth specifically cited Google and Apple as powerful forces that Twitter must contend with in terms of how it handles its content moderation. 

“Failure to adhere to Apple’s and Google’s guidelines would be catastrophic, risking Twitter’s expulsion from their app stores and making it more difficult for billions of potential users to get Twitter’s services,” Roth wrote. “This gives Apple and Google enormous power to shape the decisions Twitter makes.”

On Tuesday, in response to investor David Sacks calling Microsoft, Apple, Google, and Amazon “gatekeepers,” Musk tweeted that “it’s a real problem.”

“Apple and Google effectively control access to most of the Internet via their app stores,” he said.

Twitter did not immediately respond to Fortune’s request for comment. Apple did not immediately respond to Fortune’s request for comment.