Elon Musk isn’t feuding with President Donald Trump right now — but he’s having it out with one of his Cabinet secretaries.

“Sean Dummy is trying to kill NASA!” Musk wrote on X on Tuesday, referring to Transportation Secretary Sean Duffy.

The missive came in response to a report from The Wall Street Journal about a “power struggle” over who will permanently lead the National Aeronautics and Space Administration, or NASA, which Duffy has been leading on an acting basis since July.

According to the Journal, Jared Isaacman — a Musk ally who was previously nominated by Trump to lead the agency — is continuing to jockey behind the scenes for the role, despite his nomination being withdrawn in July. Isaacman, who has helped fund and lead multiple SpaceX launches, is the founder of payments platform Shift4.

Isaacman’s nomination was pulled amid Musk’s feud with Trump, who later called Isaacman a “blue blooded Democrat.” The billionaire CEO recently said that he believes his nomination was withdrawn because he was a “good target” in the “whole divorce” between Musk and Trump.

Isaacman, Musk, the Department of Transportation, and the White House did not immediately respond to requests for comment.

In another post on Tuesday, Musk polled his followers on whether “someone whose biggest claim to fame is climbing trees” should be “running America’s space program.”

It was an apparent reference to the fact that Duffy, a former Wisconsin congressman, was once a world champion speed climber.

“The person responsible for America’s space program can’t have a 2 digit IQ,” Musk wrote in a separate post.

Musk’s feud with Duffy also comes one day after the secretary said that SpaceX was “behind schedule” on a major NASA moon-landing project, and that he was in the process of “opening up” the contract.

In response to Duffy’s comments, the billionaire businessman said that his company was “moving like lightning compared to the rest of the space industry.”