I don't think you should mess with them unless it is causing a malfunction. The coralline algae is beneficial, and a great camouflage. If you must clean them, you can use plastic brushes to scrub them, like ones used in kitchens and cars. You can buy these at grocery stores or Walmart. Get some to dedicate to aquarium use only. If that doesn't work, you could use a steel brush or even steel wool. Don't use brillo or SOS pads that have detergents in them, just plain steel wool. You can then rinse them good, and scrub them with the plastic scrub brushes to make sure there is no steel wool left behind. The plastic brushes are cheap, and very useful for aquariums anyway. I went to the South Carolina Aquarium this weekend, where they have a 335,000 gallon tank, and they use them to scrub algae in that tank!
Sonny