HI Ronald.
I would imagine the white snails are your culprit, but there might be other causes too. So before we go with the snails, just quickly think about these things:
1) How old are your lights? With time, light intensity drops, and thus the coral might not be getting enough light.
2) Could another coral be stinging it? If you have placed it close to a torch coral for example, the torch might sting it.
3) Have you changed your flow pattern? Either increasin or decreasing the flow?
Now, if none of the above factors (and others are welcome to please add) are in check, it might be this snails. Could you possibly submit a photo of them?
I think a crustacian might help your problem, and some fish too (I am just not such a big fish expert...)
My suggestion is maybe an arrow crab. Although, depending on the species of the snail, I might be wrong. Best to maybe either google us an image or take a photo.