Ken Griffin thinks U.S. stocks are "real frothy."

The Chicago billionaire, who runs hedge fund behemoth Citadel and owns market-making giant Citadel Securities, expressed concern on Wednesday while speaking at The New York Times DealBook conference about the seemingly never-ending ride upward in equity markets that has taken place since the COVID-19 pandemic invoked a sense of terror in investors 20 months ago.

"Let's be clear, it has been a one way street since the start of the pandemic straight up," Griffin said. "We're seeing a market that is showing signs of real frothiness, where you've had some pretty significant stock price moves on relatively small events."

Stocks have indeed been on a tear since bottoming out in March 2020, with the S&P 500 climbing more than 100% in that time.

Driving the surge has been an unprecedentedly helpful Federal Reserve that has kept interest rates near rock-bottom levels and has only recently begun drawing back its support from the bond markets. The measures created a nearly two-year-long period of easy money for companies and extreme risk taking by investors in everything from cryptocurrencies to the very same meme stocks that put Griffin's market maker into the national spotlight for its role in executing individual investors' orders on behalf of popular trading apps like Robinhood. On the conspiracy theories that bubbled out of the GameStop saga, Griffin described it like being at the center of a "bad comedy joke."

For Griffin, the growing divide between share prices and fundamental values is a point of rising concern, though, as "any form of either policy error or a company having a bad spell is going to result in a pretty dramatic repricing of equities," he said.

Just take Tesla as an example, Griffin said.

The electric car maker's stock had been battered by investors following CEO Elon Musk's Twitter poll on whether he should sell 10% of his stake to pay more taxes. The survey results said Musk should sell. So, investors responded accordingly, driving Tesla shares downward and evaporating hundreds of billions of dollars worth of market value from the stock in the process.

"I never thought we'd let our stakes be dictated by a poll on Twitter," said Griffin, who added he has known Musk for a long time. "We live in a whole different world."

Crypto valuations

Equities are not alone in their precipitous climb, of course.

After a band of individual investors piled into meme stocks like GameStop and AMC earlier this year, many found their way to the cryptocurrency markets. Crypto has also become a hotbed of interest for Wall Street financial giants in recent years, including both hedge funds and speedy trading shops—both of which fall under Griffin's umbrella with Citadel and Citadel Securities.

But Griffin is still trying to make sense of the space.

"We have a variety of asset classes where the ascertaining of values [is] very difficult," Griffin said. "If you and I think about a bank, we might say, it should trade at 12-times earnings. We could debate it should be 13 or 14. But when you have to value cryptocurrencies, what it the basis that you use for valuation? And it really comes down to, 'Do I think somebody will pay me more for it tomorrow?'"

Ultimately, Griffin says there are a number of questions that crypto still has to address for it to become truly formidable like its fans believe it can be. Among them are who makes a consumer whole when their crypto wallet is stolen, how will Bitcoin reckon with its environmental footprint, and what are the "solid commercial use cases" for blockchain technology, Griffin said. Today, the Citadel CEO does not yet see any.

"People are very focused in a world of new ideas and new creations. I love that part of America," Griffin said. "I worry that some of this passion has been misplaced when it comes to cryptocurrencies."