Is that from arm tip to arm tip? That would be very small for Linckia in general. Linckia laevigata - the common blue Linckia in the hobby, gets up to a foot across, and I typically see them in an LFS easily stretching over the palm of my hand from wrist to fingertip.
Normally Linckia multiflora has some blotching...but it is considerably smaller (about 5" across), and much "hardier" in tanks like yours.
Has this ever "dropped arms" which is the standard method of reproduction?
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