To be honest, I am a brittle/serpentstar person over a seastar one but they all kinda get lumped together
Your serpentstar may be getting enough to eat as it is with your general feeding of the tank. Most - well at least many- brittle/serpentstars (I use them interchangeably) do hide most of the time during the day. They are nocturnal. Coming out during the day exposes them to some of their primary predators which are visually oriented predators. So you may see arms or so but that is often it. Some more "bold" brittlestars such as the green don't seem to mind too much when they smell food but do stay under rocks a lot - or close to them - except at night. You may try coming out after your lights go out and see if the serpent is wandering around.
When you feed, do you see any arm exposed? You can get food to the arm tip and it may take it...but it may not come all the way out. I have several that I don't see very much at all. I have 9 brittlestars in my 45g and the casual observer (and often me) have trouble seeing the majority - and even then it is usually an arm or something :yes:
So I think your serpent sounds rather normal