David Tepper is nervous

FORTUNE -- The nation's highest paid hedge fund manager is concerned about the market.

"There are times to make money," says David Tepper, who runs Appaloosa Management. "This is a time to not lose money. I think it is a nervous time."

On Wednesday, Tepper cautioned fellow fund managers and other attendees of this year's SALT Investing Conference in Las Vegas. Tepper, who rarely talks about the market publicly, was recently named the highest paid hedge fund manager by Institutional Investor's Alpha magazine. The magazine said he made $3.5 billion last year.

Tepper's biggest concerns hinge on economic growth prospects and its effect on stock prices. He said his opinion would be different if the economy was growing at 4%. But he said that, even adjusting for the weather, the economy looks to be growing much more slowly than he expected. Indeed, U.S. GDP grew by 0.1% in the first quarter of 2014.

That's a problem, Tepper says, because stocks on average are trading at 16 times next year's expected earnings. That means investors are expecting relatively strong bottom lines. But if the economy is growing more slowly than expected, profits are likely to disappoint.

Tepper says he is also concerned about deflation, given the sluggish economic growth prospects. "If we have price pressure as well, then that's really going to push down profits," says Tepper.

Tepper says that while he is normally seen as a bullish investor, he currently has a portion of his money in cash. "I'm not saying go short," Tepper says, which is the Wall Street term for betting against the market. "But don't be super long either."