A small shell has drilled into the cloth of the cap by it's brawny perforator leg.
Identification by exclusion showed that this is probably Lepas anserifera goose barnacle of the infraclass Cirripedia.
Legs of the crustacean living inside the shell have transformed to feather-like thin processes. The barnacle puts them out of the shell waiting for enough detritus to stick, then pulls back inside and licks them clean. Generally, the manner of nutrition similar to that of the polychaeta.
For a long time, both goose barnacles and sea acorns were considered as mollusca because of their shell and sessile mode of life.