What kind of animal is this?

Dear Patrizia,
Normally I try and avoid answering questions on things that are clearly not a sea slugs, simply because I don't have time for all the sea slug queries, but since your object is the food of a sea slug, I will be kind.

The white object in the middle could be a bird skull, but what ever it is, it is just a floating object for the other objects to attach to. Eac of them is a separate animal, a barnacle, attached by its leathery stalk to the white floating object. From the different sizes of barnacle you can see in the photo, larvae have settled on the float a number of times. Barnacles are very specialised crustacea [crab, prawns, lobsters etc], and this stalked group are often called Goose Barnacles becasue of their resemblance to the head and neck of a goose. In fact in medieval times they were said to turn into geese - though I can't quite see how medieval people would have missed the point of eggs.

If you look at the Fact Sheet for the nudibranch Fiona pinnata, you will see photos of live goose barnacles, which are its preferred food.

Best wishes,
Bill Rudman