Can't a sea slug have a few secrets? Hero formosa is found in relatively shallow waters in the UK and Europe, most recently from the west coast of Scotland. Not the most remote marine habitat for curious humans; still, this species is relatively scarce, so there's a lot we don't know about it, like, for instance, what it eats. It is often found on or near hydroids, which are considered tasty by many other nudibranchs, so that's a possibility. It'll probably take a few more careful, and lucky, dives to find out for sure. More tidbits of information here.