It's probably algae growth. Take it out and scrub it with an algae scrub pad (found at any pet store) under warm tap water - like Meatpuppet said, DO NOT use any kind of soap or chemical. To keep this from happening in the future make sure you clean tank decorations regularly (just do the same thing with all the decorations as I just told you to do with the skeleton). You can also use the algae scrub pad to clean algae off the glass of the aquarium - don't worry if it breaks up the algae and it floats around, your filter should catch any pieces of it that break off.
Other ways to prevent algae - don't leave the aquarium light on for more than 10 hours a day, make sure you so partial water changes weekly, and don't put the tank in direct sunlight. If you ever have a huge algae outbreak you can buy something called AlgaeFix that will kill algae and help keep future growth down.