I really doubt he ate your hermit - the diet's of Linckia's aren't 100% known, but as far as we understand, they consume an algal/bacterial "film" off LR
. This is why it is usually recommended that they be kept in a fairly large/mature tank - a lot of LR
is very important.
I don't think they are truly hard to keep - as long as you have enough LR
that they don't starve, and you keep your water parameters stable. Almost all failures with these animals are due to either (a) starvation, or (b) their insensitivity to changes in pH/salinity/temp fluctuations - most of these stars that do die in the first few weeks are victims of poor acclimation.
We've had our blue Linckia for just over a year. It seems very healthy, and while it usually is pretty active, I've seen it stay in the same spot for 2-3 days at a time.