The only reason your hermits are eating your snails is because they have fallen off the glass and are laying upside down. I have never seen a blue-leg hermits actually attack a snail and pull it off the glass. Occasionally they will flip them around on the sand, but if the snail is healthy it can protect itself with its cover.

I haven't seen them eat the cyano, but I have seen emerald crabs got to town on it.