I have kept serpent stars for years, they hide a lot. You will see an arm sticking out of a rock. The ones I have almost reach about 24 inches across. The big one will come out, and I can feed it raw shrimp from super market. The second it touches the water no matter where he is hiding he comes out.
Chocolate stars might eat corals. Serpents that I have had will NOT !
And of course I have plenty of Asteria stars !