Gastropoda

I collected some marshy lake water and gunk for a "jar-arium" a few months ago. I noticed this flat symmetrical little guy the other day on the glass for the first time. He was somewhere between 0.5 to 1 centimeters long and was no where to be found the next day, meaning he's mobile. What could it be?

What is this plant called? It looks like shrimp heaven!