Vinegar kelp.

But it is not kelp. The two look similar, but there are differences if you look closely. Laminaria is a perennial large brown algae with leathery texture. The algal body is clearly divided into root-like anchors, handles and slices. It is olive brown when mature and dark brown when dry. The slice is long and narrow, with an entire edge, up to 6m long and 20~50cm wide. It is thicker in the center, thinner towards both edges, and has wavy folds.