Green Moray's can get to be over 7ft. The aquarium by us has one that was sacrificed by a private owner. It is easily 6+ ft, I've never even seen it fully out of it's cave. THEY GET HUGE.
I also believe that Green Moray's are fast growers and this can pose a real problem for people who buy a small juvenile and it quickly outgrows the tank. Unless you have 400+ gallons, I definitely do not recommend a Green Moray.
A Snowflake eel or a Jeweled Moray would be better suited for a 55. In all honestly, the Jeweled Moray is pushing it IMO.