Yes, it does sound like the Astrea snail in question has expired. Nassarius snails are more carnivorous in their dining habits and do a good job of cleaning up meatier leftovers like frozen Mysis, so it was probably one of the nassarius snails that was attracted to the dead or dying Astrea snail.
Small snails are not particularly long-lived and if you have a cleanup crew consisting of several dozen assorted snails, it’s not unusual for one of them to die of natural causes now and then. So if the death of the Astrea snail is just an isolated incident and your water quality is good, then it is nothing to be overly concerned about.
Best wishes with all your fishes!