Although we try to screen this material out sometimes a piece or two will make it into a bag. It is actually a natural component of the kelp plant and safe to ingest.
During the reproductive phase, the gas bladders of the Ascophyllum nodosum fill with water. This is what causes the fibrous material that is sometimes seen in the finished product.
The structure of the interior of the airbladder is described as “a meshwork of web-like medullary cells” (Dromgoogle 1990).