I can't add anything helpful but I can tell a story about my grey with dark strip serpent starfish. I obtained one about 9 inchs long tip-to-tip. It had been doing great in my cube, no problems, until I added my first shrimp. I added a cleaner shrimp after acclimating it. I watched it scurry away between some rocks, but when I turned around to look again, the serpent star had pinned the shrimp from above and coiled it up with it's tentacles!
I tried to get my hand between the rocks to let him go, but couldn't reach. The starfish was trying to crawl away while playing tug-of war with me and the shrimp wrapped around one tentacle.
I read up somewhere that starfish will raise their bodies up with their legs and wait for prey to come under them. They then drop their bodies over the prey and coil up on them. Guess mine perched up on the rocks. and was hungry?