Well, I can’t help with the tank size, sand, reef safeness, or other things that way, but food wise you could try putting it in its own tank and offering it a variety of food options for a while to just see what it accepts (it would probably need to be in its own tank for this to work, otherwise the other fish might beat it to the food). You’d need to be careful with the water quality, but you could try offering it somethings like a piece of shrimp, a clam on half shell, a nori sheet, some fish pellets/flakes, some fish or urchin roe (fish eggs/urchin gonads), etc. in varying sizes (some creatures only eat tiny chunks, some like big, etc.) and just see what all it eats (if anything). If it doesn’t seem to take any of those you can always try tying the nori onto the rocks or offering it some gel food pressed into a coral skeleton (an sps skeleton would probably be my recommendation) like they do for corallivorous butterflyfish.
No guarantees that any of this will work (I’ve heard of some starfish basically needing to be force fed before they start taking any foods) or that any of this even if accepted would give the starfish a long, healthy life, but it’s the best place to start that I can think of. If you do find something/somethings that it eats, make a note of it and be sure to add some to the main tank to feed it when you move it back over. Also, if you do find something it eats, keep an eye on it when you move it back to the main tank to make sure it is actually getting some food and not starving to death because it’s being outcompeted for the food it will eat.
As a side note, a lot of starfish will start dropping legs when they’re starving, so if it’s not working you might get some heads up to try and turn things around before it dies (no guarantee, as this may depend on species). Additionally, as I understand it, some starfish are detritivores, so they might not have any particular diet/food preference to meet, but they might need more food than is being provided by the tank (for example, I’ve heard of some stars that basically clear out the entire sand bed then starve because there isn’t enough detritus - like urchins that eat all the algae in the tank and then starve). Just some food for thought.
Hope this helps, and best of luck with keeping it!
