Mucous comes of body in pieces
Large sloughs of mucous come away from the body of the fish.
Protozoan Marine parasite, viz Brooklynella hostilis.
Causes major losses in many marine exotic fish imports. Is very hard to correctly diagnose. Not easy to treat but perseverance will usually overcome the infestation.
Quarantining all new fish, for at least 2 weeks is the best way to prevent spread of this( and other) infestations.