So, if you look at the abstract, they do mention underwater lichens. So they can grow underwater which solves that question. They definatly look like lichens to me, im going with that. They look neat, I would leave them, I dont think they can harm anything. You could try to scrape them off, but if they are anything like terrestrial lichens then good luck.
I was asking where the rocks came from because if they came from the woods or a stream bed the chance of lichens being transferred is much greater than if you bought them at a garden shop.
Lichens are a symbiotic (cooperative living) relationship between an algae and a fungi.