What is it?

[BettyJ]

These clam like creatures are growing on my rock. As my algae overgrowth has recessed these appeared and now seem to be reproducing. There could be 5 of them now. I don’t know if they came in on my rock nearly 5 years ago or if they came with the mushroom coral I bought over a year ago.

[BettyJ]

Here are my pictures.

[Sniperhank]

Well, sounds like simple filter feeders that are enjoying the excess nutrients in the water column. Will fade away as the nutrients do.