I also have tons of these little snails and they are harmless. I asked what they were at one time but I forgot the answer. All I remember is that I was told they are harmless and many people have them.
The new overflow looks so much better then the getto drywall mud box that you had in there. I would have bought a real overflow before I put that ugly pink/red box in my tank. I guess you now know why you should not use silicon on plastic (it will not hold). The new box will eventually fail in...