Snail - Friend or Foe

Picked this little fellow up July lst in Langley, B.C. I thought the shell would look nice in my garden shell collection. Hubby packed in his pocket until we got back to our sons house. The shell made the trip back to Merritt, B. C. in a plastic bag of scrap booking paper (I didn't want to break it). I put it in my garden with the other shells, looked great. The next morning it was gone. Did my cats find it, No it had crawled under the rock. ITS ALIVE. Now my question is "Is this cute little shell going to harm my garden?"
Help anyone