Billionaire supporters of President Donald Trump—including Elon Musk—spoke out Monday morning and issued warnings about the economy as global stock markets plunged further in response to the escalating trade war.

"A PhD in Econ from Harvard is a bad thing, not a good thing," Musk said of Peter Navarro's academic qualifications.

Navarro, widely seen as the architect of Trump's tariff plans, told CNBC Musk had done a good job with his work to streamline government, but his opposition to duties was not surprising, the latest salvo in a growing feud between the Trump advisers.