"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."
Last month, the activist investor disclosed a 7.5 percent stake in snack and beverage giant Mondelez, worth about $5.5 billion at the time.
Shortly after that, Warren Buffett told CNBC a deal between Mondelez and Kraft Heinz was unlikely in the near term. Mondelez spun off from Kraft Foods in 2012.
Ackman said Friday that Mondelez could be a buyout candidate at some point. "If you had to make a list of companies that Buffett would like to own, Mondelez is certainly on the list," he said.
Hedge funder Nelson Peltz of Trian Partners was named to the Mondelez board last year, after taking a stake.
Asked whether he'd seek a board seat, Ackman was cagey, saying "You never know."
"The company is working hard. And Nelson, I'm sure, is part of it," he continued. "We like things where there are lots of ways to make money. One way to make money here is the company remains a standalone business and gets to its potential and will do very, very well."
"Either the current team will get the business to its potential in a reasonablly rapid fashion or it will be a [takeover] target," he added.
Ackman also talked about his well-publicized short of Herbalife. He said it was not a bet only on what federal regulators might do. "I think that we have multiple regulators here that understand there's a problem."
"The last way we win here is just basic business deterioration. The company's business is declining double-digits in pretty much every market around the world, except for China," he said.
In 2012, CNBC broke the news on Ackman's then-$1 billion Herbalife short. He's campaigned ever since on his contention that Herbalife was a pyramid scheme, an assertion the nutritional supplement company has consistently denied.
"If we're wrong, we fold up the tent and we move on," he said. "But as I've said from the very beginning on this one, and I would say it's higher today, the certainty factor here is the highest it can be for an investment."
Herbalife was not immediately available to respond to CNBC's request for comment.
Valeant did not end-up buying Allergan, which was eventually bought by Actavis last year, in a $66 billion deal that netted Ackman $2.2 billion in paper profits.
"Every day you read about deals that don't happen." Ackman said Friday. "It's very likely we'll partner with another strategic to make a bid for a business."
"We may not get it," he continued. "But if the outcome for shareholders is the business either improves on its own or it gets sold to someone else, I think that's a win."
Another Ackman holding, Canadian Pacific Railway, has fallen on hard times this year on slumping oil prices, which caused a decline in energy shipments and a recession in Canada.
But since Pershing started acquiring its activist position four years ago in CP at $44 a share, the stock has surged to $145 as of Thursday's close.
"It's stronger. It's more profitable," Ackman told CNBC, sitting next to the CEO he helped put in place to turn around the railroad.
Canadian Pacific chief Hunter Harrison said, "We're not traders or investors, we're railroaders. And what we do ... is try to provide the best service for the customer at the lowest cost. Let the markets work out what the markets work out."
"I told you all a lot of things three years ago [and] nobody believed me. We had a horrible reputation," he said. "I think we've proved people wrong."
There's been speculation that CP might merge with U.S.-based CSX. That company's heir-apparent, Oscar Munoz, became CEO at United Continental this week, a job vacated by Jeff Smisek amid a probe into improprieties at the Port Authority of New York and New Jersey.
Canadian Pacific COO Keith Creel, who also appeared with Ackman on "Squawk Box," said he would not comment on speculation. "Obviously there's an opportunity there. But it's up to CSX to decide at this point if they're aboard," said Creel, who's due to take over as CEO at CP in less than two years when Harrison retires.
Correction: An earlier version misspelled Valeant in one reference.