So what do you do? Most people will get out a brush of some kind and scrub it while holding it under running water. That will do a partial job. So how do you get it back to the crisp white color that it was when you bought it? A tip I picked up from a fellow cichlidophile is that you sun bleach it. No need to take a brush to it. Just set it out in direct sunlight for a couple of days. The longer it’s exposed to bright sunlight, the whiter it will get (to a limit). A week ago, the rock on the right in the photo looked almost as bad as the one on the left. Notice the difference after a few days in the sun.
If you’ve kept African cichlids, especially mbuna, you’ve probably used holey rock at one time or another. I’d be willing to bet that you’ve also had rock in which its exposed surface within the tank has accumulated diatomaceous algae (the brown stuff) or regular green algae. Either way, you’ve probably also discovered that such algae is not the easiest stuff to remove. The surface of holey rock, unlike river rock, is usually quite course, which makes it difficult to clean.