Well, it really depends on the star, and potentially whether it is stressed or not. In general, healthy brittlestars (serpentstars are the same animal) will seek cover immediately. The tend not to come out in the open during the day, instead the extend their arms out from crevices or from under rocks. In the wild, they might also hang out in large sponges. So if the serpent is out in the open, all day, I am a bit concerned.
Some would seek this sort of 'perch' in the wild if they were filter feeders, but again, not usually during the day. And since you are describing it as a serpent star, it woould not likely be a filter feeder.
Now to correct a myth, they should not be left simply to scavenge for debris. They will not stir or sift the sand, except through hiding under rocks. I don't know why people have come to that idea. They should be spot fed, something meaty like shrimp pellets, krill, squid, etc. They will clean up flake food ect, but this tends not to be enough food. This activity typically occurs after lights out. Brittles show a strong dislike for bright lights. In general, they are not often seen except after dark, or if there is food in the tank.
How did you acclimate the star? If food (like a piece of krill) is held nearby, does it move the arms in an effort to locate the direction of the odor?
