According to your description, it is probably a sea rabbit. Also known as sea slugs. This is a cephalopod. Fighting slugs on land. There are also many varieties that are very similar in appearance. The shell degenerated into a body. Generally feed on seaweed or kelp.

Colorful in color and appearance. At present, the understanding of it is not complete. The mouth on the back should have gills. When stimulated, the mouth of a sea rabbit contracts its gills.