That depends on if the snail is feeding off of the fish. I may well be wrong but I think I'd want the snail off of the fish ASAP. Actually I sort of wonder how a snail was able to attach to the beta in the first place....Personally, I think you will add more stress trying to get it off. It will come off on its own soon, I suspect.
Are you sure it's a snail? It must be a very brave snail to hitch a ride on a betta, or on any fish, unless it's a dead fish.Is it OK if a snail is attached to my crowntail betta or should I try to knock it off