Colonial hydroids. Remove the snail from the tank ASAP. Personally, I wouldn't risk trying to remove them from the shell as you likely won't get the entire animal(s) and they'll just reappear. If you are able to humanely euthanize the snail, that's the way to go unfortunately...
You could pull snail, keep it in seperate container and epoxy the shell for about a monthto starve them. Then pull the epoxy off if trying to be humane as well if you cant find an etgical way or want to kill the snail.