I found some giant kelp coated with a material that is white, smooth, and forms a chain-mail pattern with small black oblong lumps in between the links. It feels pretty hard but not stiff (the kelp still flexes), and can't be easily removed from the kelp skin. The leaves under the coating appear to be stunted and malformed.
This was found in the Pacific in Southern California in the late summer.
Is it a parasite?