Execs in any public company routinely sell shares at a pre-determined rate every quarter. Execs besides Elon have been selling as part of the plan, and company employees are free to sell as well.
According to SEC filings, Elon himself hasn't sold any recently, but he owns 20% of Tesla, worth about $10B today. Even if Tesla drops 10x, he would have $100M worth of shares. Not to mention all his other assets, shares in SpaceX etc. He has more money than he will ever need.