?

My mystery snail has been fine for about a month in my 15 gallon tank. I also have 3 new black skirt barbs and 1 swordtail fish who has been in their for the same length as my snail. I also have 4 shrimp. None of my other fish seem to be bothering him. Sometime he will go half way into his shell and lay on his shell, with his body up in the water. I took him out to see if he was dead and he was not. Does anyone know what is wrong with him and what I can do to get him better? And what are the white spots on the swirl of his shell?