"Stocks are pretty cheap," Ackman told CNBC's "Squawk Box," a day after Tepper said on the show that he's "not as bullish" as he could be on U.S. equities.
"Currency [fluctuations] have been largely reflected in next year's [earnings] estimates," Ackman said.
Caught up in the stock market turmoil on concerns about China's slowing economy, Ackman's Pershing Square Capital, with $19 billion in assets under management, lost 9.2 percent last month, erasing gains for the year.
"Talk to anyone who knows anything about China [growth], they'll tell you that 7 percent is fiction, 5 percent is probably a fiction, and real growth in China is something like 1 percent to 4 percent," he said.
Last year, the Pershing Square boss was the top-performing hedge fund with a return of more than 40 percent.
But Ackman said he's not sure how the rest of 2015 will shake out.
"We're principally a long-only fund, with the exception of Herbalife [short]. We own concentrated positions in companies. We own them for years," he said.
"I can't tell you where any of our stocks are going to be next month. I can't even tell you where they are going to be at the end of the year," he continued. "Mondelez will be a much more valuable company a year from now than it is today. Two years from now, it's going to be even more valuable."