ya i haven't added anything to my tank for years...... nor have I done a water change in probably 2 years. The softies hate it when I do a water change so I just left it and I am getting great growth. Weird stuff just kind of shows up lol. It's killing me to not know what it is as well. I'm sure its just some common oyster or clam or something. It would be cool if its something rare though
I had a roughly 8" abalone show up one day as well about 2 years ago from behind the rockwork. He ended up dying and I have a pretty good size shell off of him.
I would go in after it, but I have like a 2 foot toadstool that I don't want to make angry. He's kind of my prized possession honestly as dumb as that sounds. But he sits right on top of it and I don't really have a place to put him while I move everything around to get to it.
I have a snail as well that is about the size of my fist that I've had for I bet 10 years at least lol. He actually has the power to eat the coraline off the glass haha!