First of all, you have to consider the fact that sometimes tank inhabitants just die. The stress of the aquarium is inavoidable even if you can keep it within reason, and occassionally they just die for reasons beyond our control. However, I would say that a good percentage of the time inverts die from starvation because people buy too many of them or they buy a bunch to rid the tank of something and then when it's gone they starve. However, I don't think that's your case since you don't have a lot of inverts and you have a fairly sizable tank. It could also have been a bad molt that caused it. You never know. I wouldn't worry about it.