so collecting shells at the beach and putting them in my tank as decorations will help?Rock identification isn’t my specialty, but “beachrock” (see the link below) may work, corals that are totally white and dead, sea shells, etc. are all calcium carbonate and should help (these are also known to grow algae pretty well). A better option would likely be to just buy some dry rock from an aquarium store, but these other options should all theoretically help.
The construction of an amazing island like Tetiaroa by the chemical wizardry of animals and plants combined with the actions of the ocean has to be one of the great wonders of the world.www.tetiaroasociety.org
The smaller pattern may be a bryozoan or similar, simple life form.