Also keep in mind that if you have not had them at least 18 months, you have no way at all of knowing how healthy they are. They can take an extremely long period to starve 8 to 12 months easily, after digesting their internal organs, and you will know nothing. Most people think they die from being attacked by something, but in fact, they starve and start to disintegrate, losing large sections of their arms, and then just being gone.
They have NO need for LR rock really, except that it may help get critters into a sand bed. Unlike Linckia, these types of stars need wide open expanses of sand, not rock. And they definitely do not eat algae, nor do they typically scavenge food or pellets ,or eat detritus. They ARE eating the critters that do, however.