Asterina starfish are common hitchhikers in our aquariums. You may see a few, or you may watch them grow into the hundreds over time. Usually they become an eyesore when seen in abundance, and you may spot them on the rockwork, the glass or even on corals or molluscs.
Some asterinas may become problematic if observed eating SPS coral or Zoanthids, although that is not their normal food source. Easily removed with tweezers if desired. For serious infestation levels, a Harlequin shrimp will dine upon them and reduce their numbers.
In this picture, the stomach of the starfish has pushed out against the glass to consume some film algae.
Asterinas propagate by tearing themselves in half.
You can see one in relation to other tiny specks of life in this next image, in the lower right corner. The white circular items are spirorbid worms. The oval shaped item in the center of the image is a Keyhole Limpet. The purple veins in the lower left section is coralline algae beginning to grow.