Just a generic bivalve. Pretty common hitchhiker. I have a bunch, one is two inches across now. You might get lucky and it turns out to be something pretty, but it'll probably just be brown. IME very slow growers. It's taken ten years for the big one I've got to reach 2 inches.
They are surprisingly mobile. And they can reproduce in the tank - I assume either something eats them or the babies are starving to death. I think I would have a lot more than I do if it was easy for them to breed, but I definitely see new tiny ones popping up occasionally. They aren't as nasty as the freshwater clams that can clog your sewer and storm drains. Think they grow too slow to cause any damage, but in that same vein, aren't some miracle cure for dirty water either.