Sea cucumbers not taste aquatic hunters rather, why they are not loved to eat. The Pearl Fish contrast, is good eating. For us he looks not absolutely delicious, hunting for underwater Animals but obviously already. But as caves underwater are rather scarce, this Pealfish hidden like in Ass End hole of a sea cucumber. He then chills a bit, to him is boring – then he invites a few friends and family, because so cozy body cavity so common is much nicer. In German the Pearl Fish is incidentally called pearlfish. That's why, much white also Wikipedia. At 0:35 one can see a sea cucumber sometimes poop, if someone is interested for… ;)

Nails
Pearlfish hides inside a sea cucumber - Natural World 2016: Episode 2 Preview - BBC Two