|Not sure what it needs eyes for|
For the first several months of my tank, some kind of rapid-growing mollusc would start to appear on the glass sides here and there, occasionally expanding to an impressive size in a very short period of time. Their shells are very flat and adhere tightly even to glass, though they could be loosened and slid along the glass – to get them out of the way for a photo of something else, for example. Web searches under "barnacles" and "limpets" and such were turning up nothing.
And that was because I was looking in the wrong place. Receiving a list of species that some potential clients were looking for, I had to look up what several of them were, including "slipper snail," and suddenly I had an answer to both questions. It's hard to place an appearance like this with most people's idea of a snail, but that's marine biology for you.
This one is an eastern white slipper snail (Crepidula cf. plana), common in the Indian River Lagoon. It feeds by extending a proboscis and siphoning food from the surrounding water. In this photo of a backlit juvenile, where the shell is still thin enough to pass decent light, you can get some idea of the anatomy. The large tan-brown oval is the 'foot' which adheres to the surface. A peek at the typical outside appearance can be seen here.