And people wonder why newbys never stick around...
The material is celluloid. Soap and water is probably the best, but you have to know that it is an unstable material that will eventually self destruct, outgassing toxic material as it does. It will rust your steel and other materials. It should NEVER be stored with anything metal you want to keep unblemished. Museums keep celluloid objects in the freezer to preserve them.
If you notice any rust, and shrinkage of the scales, any dullness in the finish, any warping, etc, than it has started it's decomposition and cannot be saved. The only way to save the knife (or any other object with celluloid in it's assembly) is to remove that material and replace it with a modern plastic or (in the case of a knife) wood, bone, micarta, or another stable material.