Linckia are quite tempting and beautiful but unfortunately most will die. And not of disease. They die of two things: acclimation shock (within a month or so of introduction or ANY major fluctuation in water parameters, esp salinity and pH) and starvation (9-12 months). Generally a very long acclimation is required, upwards of 5 hours, depending on salinity, etc.
They do not eat algae and can not be spot fed. Rarely will they eat anything meaty. I've never known one to take mysid shrimp or be attracted to spot feeding (if they did, people would have much more success...as it stands, there is rather little long term success). As mentioned, they require huge amounts of LR to have a chance of survival, and generally in larger tanks. Large mature reef tanks seems to be a necessity.
IMO. They also should not be introduced into a system less than 6 months old. The specific gravity should be around 1.026. If too low, they do not do well. Watch pH and alk as well.
You really can not keep two Linckia sp stars in a 75g tank. They compete for the same food, and it takes time to replenish it. The diet may be quite specific but it most likely includes algal and bacterial films, sponges, etc. No one really has a clear idea, and that includes seastar specialists.
Unfortunately, one can not claim 'success' with a blue Linckia, or really any Linckia, until it has been kept over a year.
The only exception, if you didn't have the blue Linckia, would be Linckia multiflora which does better in captivity. But if you have the Blue, that is all you will really be able to keep. If it does not survive, I beg you to please read more on the requirements of these seastars before buying any others. Many are really tough to keep and it is throwing money away in many cases to buy them (think your LFS spends much time acclimating them!?).