Yes, that was what happened. Because I did not see it since it was dead behind the DW & rocks I didn't realize until after the 2nd day the shrimp were mostly dead and dying, some made it the last time in the 15G tank was the large snail.
Now, I make sure the Mystery snail are moved out when they are proportionally sized in the small tank. Unless they were tiny like green pea to Garbanzo bean sized babies, I wouldn't put any in under 5G any more
. I use one Nerite (too many and they seem to run out of food and die off) or some Ramshorn for smaller tank.