Chiton

Chitons are mollusc from class Polyplacophora , The name come from Greek word khiton "frock (worn by both sexes), tunic, mail coat" (chitin). Used in English in literal sense of "ancient Greek tunic" from 1850. The molluscs also are known as "coat-of-mail shells" for their mail-like covering.

The photo is showing a little creature hiding behind relatively big red sea anemone. On the second photo can be seen its really small size. It is not so common around this area, although I don't remember its change in occurrence.