Elon Musk didn’t mince words after Robert Reich, the Clinton-era labor secretary who now researches public policy at the University of California, Berkeley, criticized the Tesla CEO over pay cuts earlier this year and called him a “modern-day robber baron.”

“All Tesla workers also get stock, so their compensation increased proportionately,” Musk said on Twitter Wednesday in a response to Reich.

Musk’s net worth plunged as much as $16.3 billion Tuesday, the largest single-day decline in the history of the Bloomberg Billionaires Index, as Tesla shares fell more than 20% before rebounding 6% on Wednesday. Even with this week’s rout, the company’s shares have more than tripled since the April pay cuts.