I have a few blue legged hermits as well and I have been having a dissappearing snail problem. I couldn't figure it out at first but I believe I have found the culpret. I didn't think it was my hermits because I have watched them crawl right over my snails and never even attempt to eat them. Well twice now I have found my smallest hermit attacking snails. I have been going down in the middle of the night just to check and there he is eating another snail. He doesn't even take their shells
. I can move him to the other side of the tank and within minutes he is back at it. I have started feeding him pieces of meaty shrimp like krill and so far so good. I guess he is just like a little boy who is growing and he is just always hungry. Luckily snails don't cost that much so I can always replenish my supply. I would suggest checking things out at night and just see what you find.