Yep, that’s right: even algae can grow a shell if the right chance mutations happen. In seawater, there’s a certain amount of dissolved calcium and algal coral luckily evolved a mechanism for ‘fixing’ calcium to carbon and oxygen to express calcium carbonate.

On any patch of exposed rock in shallow water, there’s likely to be thin layer of crusty, reddish, stuff. That’s an algae, hiding inside a shell. That’s you.

It doesn’t make huge reefs, like polyp coral, but given enough time, it probably would. Two utterly different species, kingdoms apart, have evolved exactly the same mechanism to solve the same problem.