Nurdles are tiny plastic pellets from the plastic moulding industry. Raw plastic is formed into pre-production plastic pellets or plastic resin pellets less than 5mm in size. Their size and shape make them easy to feed into the hopper of a plastic moulding machine. Not every nurdle makes it into the hopper. Some may be spilt or blown away on the wind. These usually end up in waterways.
Nurdles are also known as mermaid tears because they make up a large portion of marine debris. Scientists have been aware of the danger of marine plastic debris for some years now*. However it is only the recent set of images of dead albatross chicks that have made a wave in the sea of public consciousness.
*So have I (see blog entry here) though not as long as they have obviously.