I remember sea silk (made from seaweed) back in the day. I think the one I knit with was made on the Atlantic coast of canada. Amazing textile, very similar feel to silk. It disappeared about the same time as milk silk, corn silk, soy silk, bamboo silk, and the like got renamed rayon (because that's how it's made). The government is fussy about accuracy in textile lables here (for which i am glad). I also heard there was another traditional textile called 'sea silk' but didn't know what it was at the time.
The article in the original post of this thread says "another, much rarer, cloth known as sea silk or byssus, which comes from a clam."
This byssus looks amazing. What a wonderful labour of love!