From the comments:

“A scientific paper titled “Interspecific Communicative and Coordinated Hunting between Groupers and Giant Moray Eels in the Red Sea” shows that moray eels can form meaningful, mutually beneficial interspecies relationships. This explains why this eel, though shy at first, eventually accepted the humanfish as a possibly beneficial partner. When the humanfish brought some prey, and never threatened the eel, the eel decided to accept the relationship. Getting grooming (scratches) is a bonus!”

Still wouldn’t try this at home, kids! 🙂