I think we're talking about the same fish under two different "common" names. Psychedelic and Green Mandarins are the same, I believe.
Anywho, regardless, everything I have read indicates they are diurnal fish that go into camo mode at night and stay very still, which acolotto described. This is probably why you don't see yours at night, crabs.
Side note, I'm picking one up in a couple days and am super excited about it!
Someone stop me! Please! But... not just yet...
55 Gal Tropical FW
55 Gal Mixed Reef