The late-Wednesday quip came after a Twitter user alerted him to a CNBC interview in which the 64-year-old Microsoft co-founder took issue with Musk’s attitude towards the coronavirus pandemic, which Musk called “dumb” in March.

“Elon’s positioning is to maintain a high level of outrageous comments,” Gates said in the interview. “He’s not much involved in vaccines. He makes a great electric car. His rockets work well, so he’s allowed to say these things. I hope that he doesn’t confuse areas he’s not involved in too much.”

The pinned “Billy G” tweet on Musk’s page, which has 37.2 million followers, is surrounded by musical notes, a reference to Michael Jackson’s 1982 hit song “Billy Jean”.

Musk has been outspoken about the pandemic, saying on multiple occasions — including on a podcast interview with Joe Rogan — that false positives and incorrectly attributed causes of death were blowing concern over the virus out of proportion.

He railed against California health officials in May when Tesla’s Fremont factory was forced to shut down its production line in May under the state’s lockdown orders, eventually daring the state to arrest him if it tried to intervene in the plant’s reopening.

It’s not the first time that Musk and Gates — the No. 8 and No. 2 richest men in the world, respectively, with a combined fortune of $183 billion — have butted heads.

Gates earlier this year revealed that he had purchased his first electric car, a Porsche, adding that he felt consumers still need to “overcome that range anxiety” in order for electric vehicles to hit the mainstream and help battle climate change.

“My conversations with Gates have been underwhelming tbh,” Musk responded.