As long as you don’t get a red or green brittle star, they are harmless. Brittle stars will pick a crevice and stick their legs out to catch floating food. Serpent will move about the tank trying to grab things. They are really cool to watch move and eat. Definitely a cool thing to have. I will advise you don’t get a serpent star if you have any larger crabs (like emeralds). They pick at their legs. My emerald killed my serpent.
Just got rid of my two short-spined serpent stars after I "surprised" one grabbing my only pistol shrimp from its crevice, and the other harassing almost to death two fan worms. Turned out they demanded more leftovers from the fishes than it is usual in my tank. No more ophiuroids for me...