Here is some helpful information about Asterina Starfish in a reef aquariums
What do Asterina Starfish eat?
Some Asterina Starfish will eat coral. Most eat cyanobacteria, coralline algae, tiny worms, detritus, and diatoms.
Are Asterina Starfish Good or Bad?
Most people will define it as a pest, while others see it as harmless. The fact that they eat some bad stuff may seem like a good thing for a while. You will want to get rid of them at some point. If they have enough food, they will multiply. Hundreds of them on the glass can be unsightly. Some species of Asterina Starfish can live in harmony with corals. They will start preying on corals if they lack enough other food.
Where did the Asterina Starfish come from?
You may have had them for a while, but they were hiding in the rock. They hitchhike on rock or coral.
Controlling the Asterina Starfish population
Adding one or more Six Line Wrasse is a good start. You will need to feed less, so the wrasse gets hungry. In many cases this will not be enough. The Clown Harlequin Shrimp is a predator that only eats starfish. Be aware that the shrimp will run out of starfish. If you want to keep the shrimp, you will need to feed him starfish. Some people do this by adding Chocolate Chip Starfish to the sump. They then rotate, cutting off a leg from the starfish to feed the shrimp. The starfish will regenerate their legs.