Unexpected Sea Hare Appears in Mediterranean Aquarium Ecosystem

I have a mostly Mediterranean native aquarium for macro algae and a few inverts (aside from the algae in the video which is Fucus vesiculosus) and this guy popped up yesterday. Is it a sea hare of some sort? I have no idea how long it has been living in this tank for or if it is new. I have species I have some species I collected in the Balearic islands but most were collected in the northern Adriatic. It didn’t come with the Fucus vesiculosus as my specimen originates from the Baltic Sea and was kept separately for a decent chunk of time)