The key question to ask whenever using data from an API or other source is "what licence is attached to the data?"
Quandl doesn't give any definitive answer, but it points to the original dataset's licensing.
In the T&C it reads
Quandl is an index. It is a conduit to data published on various locations on the internet. Like any search engine, Quandl makes no claim to own the data it indexes or caches. Quandl endeavours to respect copyright. If you believe this site is indexing your copyright data and you would rather it not do so, you may submit a take-down request.
Using the Website does not give you ownership or license of any intellectual property rights in the Website or in any content, information or data accessed on or through the Website, including content, information and data obtained from a third-party website.
Both are quite clear in not granting any explicit right to data re-use.