We bought a war paint goby, acclimated him to a small tank all his own, and he dove under a rock and hasn’t been seen since. A second one acclimated to a populated tank, vanished, to be discovered hiding in a duncan coral. The citron gobies hang out on the side of the tank, using their goby suction cup fins. I am amazed that two species, exactly alike save for coloration, show such different behavior.
I haven’t ventured into saltwater, but I’ve noticed the same behavior variance with different types of loaches. I got a little black one that I didn’t see again until I was changing the under gravel filter (he hung out down there), and I had a yellow one that stayed out in the plants and “hid” by wrapping himself in leaves.
Mrs. Plant’s (v.3.0) loaches tend to hide until it is time to eat, when they swarm out like ravenous…fish.
I love their little barbels around their mouth. 
Could it be having company? I don’t know from gobies, but clown loaches own the tank in groups of five or more. Fewer than that* and they dig out a cave usually underneath the cave I have provided for them) and hide all day.
- Always excepting Angel, the antisocial clown loach who eats anyone I put in the tank with her.