Those are limpets. Some eat corals. Some are just intertidal and eat algae. Keep an eye on any zoos you have. I have some in my 12. You can find them sometimes up in your filters where there is high water flow. The ones you have look like algae eaters. The small white dots on your tank corners might be tiny hard shell tube worms. Do they look like they spiral? If you look really close you can see a crown.