Gads! I read the link that was provided in your link. These things are going to be tenacious! IMO, you will have to use a sharp, fine pointed knife to scrape them off AND you must scrape off the whole tube, which may go deep into the rocks' holes in many places. I don't envy your task
What I would suggest is using an Xacto knife for scraping. Get one with a very fine blade point. Have a bucket for rinsing the rock while you work on it so you can scrape one area, rinse then go onto another area for more scraping. Then rinse each rock in another bucket before placing in a holding container for the scraped rock.
Good luck at getting them. You may have to do this a few times over the next few months to make sure you get them all.