A smaller whitemouth moray eel displays the feature that gives them the name. An eel will typically open and close its mouth in this way, but it’s not an act of aggression. It’s how they breathe, the motion pumping water over the gills.
This pose, with much or most of the eel hidden in a crevasse or coral head, is also typical. It’s how I make most of my sightings. But every so often I’ll see an eel in open water or passing from one hiding spot to the next. It’s usually a fleeting sight as eels can zip along and disappear into the tiniest of cracks.