Your tank is far to small for a blue Linckia, and probably any reef safe star.
What are your tank parameters?
How old is your tank?
Please understand that seastars are some of the most difficult animals we can keep. tank parameters must be ideal. And even then, many tanks are not suitable.
there are brittle and serpentstars that would be suitable because you can spot feed them. But most reef safe seastars are likely to starve in time.