Saw this little critter on my rock today. Not sure what it is, only snails I have in my tank are red stripe and nasarrius and I know what baby red stripes look like. Its the silver shimmer in the middle of the rock.
They start out slow, but they can reproduce fairly quick. I use to not have any, never added any, but now have them all over my sump, and some are as large as a quarter. Others as small as a pin head