I have two blue mystery snails in my 10 gallon tank. They've been doing fine lately but one of them has been sitting just under the water level and it seems like it's secreting some sort of mucus. I can't really tell what it is because it's on the back side of the tank which is against the wall. Anyone have any idea what this could be? Is it bad? I don't want a dieing/dead snail in my tank...