If I have an algae problem and need to scrape that off an ornament I would usually pull it out of the tank and run it under hot water and scrub with an algae brush, but that would only be if it got real bad. What is your purpose for cleaning the ornaments? The good bacteria that keeps the tank cycled will house itself on your ornaments so it isn't a good idea to clean them too often. If/when you do clean them, you should watch your tank for an ammonia spike. If you just have a little bit of algae on the ornament you might just clean it in a bucket of water that you've filled from the tank itself during a water change. This way you can still keep the good bacteria alive on any places that you don't scrape. Hope this helps ya out.