Josh already gave you your answer. You don't have a predator. You have stars that are dying in a very typical manner due to osmotic damage. The stars go through several steps before they get to you and failing to acclimate them properly at any step along the way can do them in. Very few exporters and wholesalers, or even retailers for that matter, take the time to drip acclimate newly arrived stars. Many shops also keep their salinities low for various reasons, so even if they are properly acclimated, issues can still arise. Of course, the same is true if you keep your salinity below about 1.024. Below that, all the careful acclimation in the world won't matter.
Also, the tank you have listed is much too small to keep any of the reef safe stars aside from serpent and brittle stars. Any that did make it past the first few weeks would likely starve to death within the first 18 months or so.