OK, here is the big question:
How long has this tank been set up?
What are your exact water chems, such as pH and salinity?
Seastars and brittlestars are incredibly sensitive to sudden changes in many parameters, and can die of osmotic shock in about a month. This makes shipping a particularly stressful time.
Linckia species stars, and their relatives, are well known for this, and require very long and precise acclimations.
Chocolate chip, red African and general stars are PREDATORY and will eat snails, corals, etc. They can be spot fed, but they most certainly will continue chowing down on anything else they desire, some of which may be algae.
Linckia sp stars are notoriously difficult to keep because their diet is unknown. They DO NOT EAT ALGAE of the sort people expect. They might eat microbial films or, indeed, sponges. They do best in very large tanks with lots and lots of LR.
Sand sifter stars, IMO, should be discouraged in this hobby. Most, unless in a tank of AT LEAST 100g minimum, will starve to death in 9-12 months. They eat the GOOD things in your sand bed and are not considered a good addition to a deep sand bed system. Most will die a slow death that is characterized by a slow disintegration of the arms.
Brittle and serpentstars can be unpredictable in their predatory behavior. Most are good scavengers as well but they do rather little in terms of "cleaning" the sand other than picking up particulates that may sit on the surface.
The only relatively good bet for tanks under 75 g in the Linckia group is Linckia multiflora which has been known to reproduce in smaller tanks (via arm drops) so long as conditions are pristine and there is lots of rock. A search should turn up some info on them.
Most other Linckia, perhaps excepting the orange, will die in a tank of this size, of starvation, in 9-12 months assuming they make it though acclimation. But personally, it is hit or miss with these guys.
These are not stars for tanks less than AT LEAST 6 months of age. Most stars are not suitable for young tanks but Linckia really stand very little chance.
One must appreciate this when it comes to "resisting" a purchase of these stars. Every shell shop in every two bit beach town and craft shop in the world seems to have them. This hobby places a huge additional strain on them. Please only attempt to keep them if you can insure that you provide a home that has a chance of keeping one alive.
Due to the rapid changes in salinity and pH, most seastars are not recommended for smaller nano tanks. If you are good about keeping up with it and the tank has been running for some time (many months), a brittlestar (aka serpentstar) is an option. Predatory stars are also options but not with corals...and they may outgrow the tank.
He is 3 inches across and has 6 legs. I'm hoping he'll drop one soon and then I'll have two.......