We don’t know what species Nagilum belonged to, and the form we see him in isn’t his true form, but he is certainly green.
Nagilum was an advanced being that lived somewhere beyond our own universe. The Enterprise D encounters Nagilum in a mysterious void in space. The Enterprise and its crew are then put through a series of trials by Nagilum as he holds them in his interstellar laboratory. Nagilum seems most interested in the fact that humans have a limited lifespan and don’t exist indefinitely. Nagilum says the he will have to kill about a third of the crew, perhaps up to one half, to truly understand death.
Picard decides to initiate the Enterprise self destruct sequence rather than let Nagilum experiment on his crew. William Riker famously says that a twenty minute timer on the self destruct makes a nice round number. In the end Nagilum releases the Enterprise realizing that Picard is not going to let him experiment on the crew.
Even though we don’t know his species Nagilum was a fascinating alien and comes in at #2 on our list.