1 - Could the snails just be dying on their own? I don't think so, They are less than a few months old.
2 - Are the shells damaged in any way?
A few of the shells seem to have holes in them on closer examination. A few of the shells look to be severly damaged. like they have been borrowed into.
3 - How long have you had the snails?
less an 3 months
4 - What are your water parameters? PH, Salinity, Temp, etc.
All water levels are fine. Salinity is at 1.022 from advice from LFS and temp is at 79 degrees.
If it is a mantis, any suggestions on how to get rid of it?
this is from c_ray0
you could do a search on mantis there are many methods some work and some dont for people. if it is a mantis look at the tank at night through a red lens to see if you can see anything in the tank that might be doing it.
Mike