Freshwater leeches are the most common that I've seen in 30+ years....I've never had any come in on snails/mysteries....but, I have caught them wild attached to plants and shrimp, and when I got a mini-colony of bloodworms to begin cultivating. It has a distinct pattern, and this isn't it. If you google it, it's got a stripe and it runs down the middle. It also moves like an inchworm....front-end then back-end, repeat...it doesn't wiggle to move...it "inches".
Now...it is possible that the wild leeches are attacking/killing the captive bred snails because most things wild have a killer instinct that captive animals can't compete with...I run and mod a bunch of groups on FB and breed certain things...mystery snails being one. They are easy enough to remove, but you don't have this. 🙂IMO, you have these, which are beetle larvae:
but then again, I could be wrong:
