For clarification here,
Echinaster (Othilia) spinulosus (and closely related species like
Echinaster (Othilia) echinophorus) in the wild are thought to primarily feeds on sponges, tunicates, biofilms, detritus, and dissolved organic matter (DOM). They can also filter nutrients directly from seawater through their skin. Anyway, they are known as both opportunistic predators and scavengers. In captivity, many sea stars - including
Echinaster (Othilia) spinulosus - are known to eat unnatural diets, consuming things like shellfish (including shrimp), bivalves (clams, mussels, oysters, etc.), small starfish (like
Aquilonastra spp. - known in the hobby as
Asterina stars, though those are species of a different genus in the
Asterinidae family), and even sand dollars.
Echinaster (Othilia) spinulosus is known to be a long-lasting species for aquariums, which indicates to me that there is a good chance they're dietary needs are more readily met by things like bivalves, unlike
Linckia spp. and similar, which tend to die off within a year even when regularly offered things like clams to eat.
Now, more to your topic here, OP: if the star has been getting consistently better (moving more, eating, etc.), that's a good sign. As long as the discoloration isn't getting worse, I'd assume the star is doing okay. The discoloration may be caused by disease, dietary deficiencies, etc. Generally, when stars start getting discolored in our aquariums, they usually basically disintegrate and die. I'm not well-versed enough in sea star diseases to know for sure what causes this/if it's related to Sea Star Wasting Disease (which is pretty well the only sea star disease that I can get Google to pull up info about), but there is an effective antibiotic treatment for SSWD (though I'm not sure what dosage or anything for it, as the info is behind a paywall). That said, though - disease or not - the discoloration doesn't seem severe enough in this case for me to feel comfortable suggesting any treatment, and any treatment at this stage may make things worse rather than better.
So, personally, I'd just keep doing what you're doing and see if it eventually improves.
Some relevant references, for anyone who's interested:
ABSTRACT. The structure of the dorsal body wall of the starfish Echinaster spinulosus was studied using polarized light microscopy of frozen tissues, scanning electron microscopy and histology. The collagen fibres of the body wall form a threedimensional orthogonal web. Voids in the web contain...
journals.biologists.com
Starfish, the Asteroidea, are the symbol of marine life. Every seashore has them, they live attached to rocks, or they crawl on the sand. They live up in the intertidal zone or out in the depth of the ocean.
gulfspecimen.org
www.marinespecies.org
