We came across a blade of kelp, Alaria, I think, in an eelgrass tidepool. Kelp must be picked up and examined. It is slimy plastic, shiny and translucent. Plus it's big.
This one had lost its grip; its holdfast had failed. A kelp's holdfast is also known as a haptera, plural for hapteron, which is a derivation of the Greek háptein--to grasp.